test: adopt native-validated 24-target probe layout

Horizontal y=56 and vertical x=38 through box 1, normal coordinates
[12.5..26.125] (root128 physical). Nearest-ACTIVE mapping with
deterministic tie rule (distance, finer level, smaller j, smaller i),
duplicate-cell rejection, stored centers and displacements. Replaces my
earlier offsets, which mapped two target pairs to the same U128 cell.
U512 spacing 128/512 and root step r -> U512 step 4r unchanged.
Tests: 24 unique cells per case, nested 3 L0 + 5 L1 + 4 L2 per transect,
U128 max displacement 0.625.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/validation/taylor_green.md

@@ -337,3 +337,125 @@ Assessment:
 - These are descriptive. Passing or failing another floor-controlled
   screen is not the conclusion, and a two-resolution ratio is not a
   convergence order.
+- Portability: the receipt's recomputed `eps = |k|^2` is compared within
+  4 ULPs (`close_ulps`), not by exact equality. A cross-runtime replay
+  differed by one ULP: 0.012047856934923534 on Horsea vs
+  0.012047856934923532 elsewhere. Source and file hashes, case identity,
+  the other metadata and every scientific threshold stay exact.
+
+### Nested shear preset (opt-in `nested_shear_n128`)
+
+A fixed three-level, two-region preset, defined in `shear_nested.py`. The
+wave, phase, Re100/U=0.03, initialization, five sample times, eps(root N),
+limits and floors are the same as before.
+
+- Refinements, given as parent-frame `(x0, y0, x1, y1)` rectangles on
+  level-wide, zero-origin unions:
+  - L0→L1: `(16,16,60,112)`, `(68,24,112,120)`
+  - L1→L2: `(44,44,108,212)`, `(148,60,212,228)`
+- Leaves `[7936, 12288, 86016]`, hops `[2048, 4096, 8192]`, work
+  771227648 per refined case; 3723493376 for all four cases.
+- Level ν `[.0384, .0768, .1536]`, τ `[.6152, .7304, .9608]`.
+
+Cases (`run_case.py --flow shear_wave --diagnostic nested_shear_n128 --re 100`):
+- `--n 128 --depth 1`
+- `--n 512 --depth 1` — the finest control is 4N, not 2N
+- `--n 128 --depth 3`
+- `--n 128 --depth 3 --explosion projected_linear [--final-snapshot]`
+
+`--final-snapshot` is allowed only for the projected depth-3 case. It writes
+`<output>.final_macro.npz`, containing:
+- ACTIVE leaves packed as `level`/`i`/`j`;
+- `rho`, `ux`, `uy` from the FP64 physical populations at the final root
+  synchronization;
+- embedded metadata: exact time, phase, mode and config.
+
+It contains no populations, restart state or time series. Its SHA-256 and
+size go into the receipt. A file over 8 MiB is removed and reported, never
+kept. All old paths stay scalar-only.
+
+Assessment: `assess_shear_wave.py U128.json U512.json R128c.json R128p.json
+--diagnostic nested_shear_n128 --output report.json --expected-revision <sha>`.
+- It reuses the case-agnostic provenance checks from the TG assessor, plus
+  exact nested scales, builds, patch, workload, metadata and the pinned
+  `shear_nested.py` hash.
+- The physics is `shear_wave.assess` generalized only in its depth-3 / 4N
+  shape contract; the metrics are unchanged.
+- `shear_wave.py` is byte-identical (81d5c8ee…), so historical receipt
+  identities and verdicts are untouched.
+- This preset changes placement and refined volume as well as depth. It
+  gives no isolated cost of an extra level and no resolution trend.
+
+### Depth-2 twin of the nested preset (opt-in `twin_shear_n128`)
+
+`shear_twin.py` uses the same two L0→L1 boxes as `shear_nested.py`, with no
+L2. Leaves are `[7936, 33792]`, hops `[2048, 4096]`, and work is 154664960 per
+refined case (611319808 for all four).
+
+The campaign is four fresh runs:
+`run_case.py --flow shear_wave --diagnostic twin_shear_n128 --re 100` with
+- `--n 128 --depth 1`
+- `--n 256 --depth 1` (the finest control is 2N)
+- `--n 128 --depth 2`
+- `--n 128 --depth 2 --explosion projected_linear`
+
+There is no snapshot option. Older exact controls serve only as consistency
+checks; they are not reused as references.
+
+Assessment uses the same preset contract as the nested preset, through
+`assess_shear_wave.py ... --diagnostic twin_shear_n128`:
+- the physics is the unchanged depth-2 / 2N `shear_wave.assess`;
+- receipts must pin `shear_twin.py`, `shear_nested.py` and `shear_wave.py`;
+- `shear_nested.py` stays byte-identical (fa6cfe87…), so the 335624c nested
+  receipts still validate.
+
+Labels:
+- *Twin vs nested* is the effect of adding this nested L2 refinement.
+  Removing L2 also changes the finest spacing and refined volume, so it does
+  not isolate abstract depth.
+- *Twin vs the single-box depth-2 runs* is a placement/geometry contrast at
+  equal depth.
+- Neither contrast is additive by construction.
+- The thin-band/acoustic explanation for the nested L1 pressure is a
+  hypothesis, not established.
+
+### Signed interface-probe traces (opt-in `--probe-trace`, mechanism diagnostic)
+
+`shear_probes.py` (pinned) uses native's validated 24-target layout through
+box 1, in root-128 physical coordinates:
+- a horizontal line y = 56, crossing the vertical faces x = 16 and 22;
+- a vertical line x = 38, crossing the horizontal faces y = 16 and 22;
+- normal coordinates 12.5, 14.5, 15.5, 16.25, 17.25, 19.25, 20.75, 21.75,
+  22.125, 23.125, 24.125, 26.125.
+
+Each target maps to the nearest ACTIVE leaf center. The deterministic tie
+rule is: distance, then finer level, then smaller j, then smaller i. This
+gives 24 unique cells in every case, and nested has 3 L0 + 5 L1 + 4 L2 per
+transect. Maximum displacement from target to center is 0.625 root cells in
+U128 (0.177 in U512).
+
+Allowed cases:
+- nested U128, nested U512, nested projected depth 3;
+- twin projected depth 2.
+
+The option can't be combined with `--final-snapshot`.
+
+Sampling:
+- The receipt stores each probe's leaf level, `i`/`j`, actual cell center
+  and displacement from its target, all in root-physical units (N=128).
+- Grids differ between cases, so centers are *not* collocated. Analytic
+  fields are evaluated at each actual center (`shear_probes.residuals`), and
+  comparisons across cases are between residuals at nearby but distinct
+  centers.
+- Times are root-physical steps 0–128 and 1920–2048. U512 is sampled every 4
+  lattice steps to match.
+- Each sample stores `rho`, `ux`, `uy` from the FP64 physical populations,
+  POST-collision G at root synchronization only. No substep content is
+  observed, and grid-scale alternation can alias.
+
+What's kept and enforced:
+- The five qualification samples stay unchanged.
+- No field dumps. The trace is validated for exact schedule, finiteness and
+  a 600 kB cap. If invalid it is dropped and an error is recorded, keeping
+  the scalar receipt.
+- There are no new pass gates; this is a mechanism diagnostic only.

tests/validation/taylor_green/assess_shear_wave.py

@@ -13,11 +13,15 @@ Taylor-Green-specific checks are replaced by shear-wave metadata and physics.
 import argparse
 import hashlib
 import json
+import math
 from pathlib import Path
 
 import assess_projected_pairs as tg
 import shear_wave as sw
-from run_case import SCALED_DIAGNOSTIC, SCALED_SHEAR, SHEAR_WAVE_SHA256
+import shear_nested as nested
+import shear_twin as twin
+from run_case import (NESTED_SHEAR, SCALED_DIAGNOSTIC, SCALED_SHEAR, SHEAR_NESTED_SHA256,
+                      SHEAR_TWIN_SHA256, SHEAR_WAVE_SHA256, TWIN_SHEAR, sample_steps)
 
 # Audited de5b0cc N128 receipts (evidence commit cba6bca, shear-wave-de5b0cc zip).
 REFERENCE_REVISION = "de5b0cc080794d0ce44d37f845aa8e8dbf8c2016"
@@ -36,11 +40,26 @@ PER_CASE = ("complete_times", "scales", "actual_builds", "reference", "runtime_s
             "patch", "workload")
 
 
+METADATA_ULPS = 4
+
+
+def close_ulps(recorded, expected, ulps=METADATA_ULPS):
+    """Portable check for recomputed float metadata only (e.g. eps = |k|^2).
+
+    The recorded value comes from another runtime/libm, so exact equality can
+    fail by one ULP (observed: 0.012047856934923534 Horsea vs ...532 here).
+    A few ULPs is far below any meaningful change; hashes, case identity and
+    scientific thresholds stay exact.
+    """
+    return (isinstance(recorded, float) and math.isfinite(recorded)
+            and abs(recorded - expected) <= ulps * math.ulp(max(abs(recorded), abs(expected))))
+
+
 def metadata_ok(record, n):
     return (record.get("flow") == "shear_wave" and record.get("mode") == list(sw.MODE)
             and record.get("polarization") == sw.E.tolist()
             and record.get("angle_deg") == sw.ANGLE_DEG and record.get("phase") == sw.PHI
-            and record.get("eps") == sw.eps(n)
+            and close_ulps(record.get("eps"), sw.eps(n))
             and record.get("numerics") == tg.NUMERICS
             and record.get("initialization") == sw.INITIALIZATION
             and record.get("limits") == sw.LIMITS
@@ -98,6 +117,126 @@ def assess(records, expected_revision=None, *, scaled=False):
     return result
 
 
+CASE_AGNOSTIC = ("finite_receipts", "same_source_and_harness", "clean_a135252_numerical_source",
+                 "same_recorded_runtime", "same_explicit_device", "same_early_sampling")
+
+
+# Fixed multi-box presets sharing one contract; only geometry, controls, pins differ.
+PRESETS = {
+    NESTED_SHEAR: dict(
+        module=nested, physics=nested.assess, snapshot=True,
+        pins={"shear_nested_sha256": SHEAR_NESTED_SHA256},
+        finest_control="uniform 4N (U512)",
+        scope="2D periodic Re100 oblique (2,1) shear wave; fixed three-level/two-region "
+              "placement; changes placement and refined volume as well as depth, so no "
+              "isolated-level cost or resolution trend"),
+    TWIN_SHEAR: dict(
+        module=twin, physics=sw.assess, snapshot=False,
+        pins={"shear_nested_sha256": SHEAR_NESTED_SHA256,
+              "shear_twin_sha256": SHEAR_TWIN_SHA256},
+        finest_control="uniform 2N (U256)",
+        scope="2D periodic Re100 oblique (2,1) shear wave; depth-2 twin of the nested "
+              "preset (same two L0->L1 boxes, no L2). Twin vs nested is the effect of adding "
+              "this nested refinement; it also changes finest spacing and refined volume, so it "
+              "does not isolate abstract depth"),
+}
+
+
+def _nested_case_ok(r, n, depth, diagnostic=NESTED_SHEAR):
+    """Per-case identity of a fixed multi-box preset (scales, builds, patch, work, pins)."""
+    preset = PRESETS[diagnostic]
+    geometry = preset["module"]
+    runtime = r.get("runtime_source", {})
+    package = Path(runtime.get("checkout", "")) / "src/CelerisLab"
+    cells = [n * n] if depth == 1 else geometry.LEAVES
+    hops = [16 * n * 2**level for level in range(depth)]
+    work = r.get("workload", {})
+    snapshot = r.get("final_snapshot")
+    expected_patch = [] if depth == 1 else geometry.REFINEMENTS
+    return {
+        "complete_times": r.get("completed") is True and tg.at_steps(
+            r.get("rows", []), sample_steps(n)[0], n),
+        "scales": (r.get("U") == sw.U and tg.finite(r.get("viscosity"))
+                   and abs(r["viscosity"] - sw.U * n / 100) < 1e-15
+                   and r.get("tau_by_level") == [0.5 + 3 * sw.U * n / 100 * 2**l
+                                                 for l in range(depth)]
+                   and r.get("mach") == sw.U * math.sqrt(3)),
+        "actual_builds": tg.valid_builds(r, n, depth),
+        "reference": (r.get("numerical_revision") == tg.NUMERICAL_REVISION
+                      and r.get("reference_harness") == tg.REFERENCE_HARNESS),
+        "runtime_source": bool(runtime.get("checkout")) and all(
+            runtime.get(k) == str(package / tail) for k, tail in
+            (("module", "__init__.py"), ("compiler", "cuda/compiler_v2.py"),
+             ("kernel_root", "lbm/kernels"))),
+        "patch": json.dumps(r.get("refinements")) == json.dumps(expected_patch),
+        "workload": (work.get("active_cells_by_level") == cells and work.get("level_hops") == hops
+                     and work.get("root_intervals") == 16 * n
+                     and work.get("active_collision_updates_by_level") == [
+                         c * h for c, h in zip(cells, hops)]
+                     and work.get("active_collision_updates") == sum(
+                         c * h for c, h in zip(cells, hops))),
+        "shear_metadata": metadata_ok(r, n),
+        "nested_module_pin": all((r.get("source") or {}).get(key) == pin
+                                 for key, pin in preset["pins"].items()),
+        # Optional, nested projected depth-3 only; bounded and hashed when present.
+        "snapshot": snapshot is None or (
+            preset["snapshot"] and depth == nested.DEPTH and r.get("explosion") == "projected_linear"
+            and snapshot.get("kept") is True and isinstance(snapshot.get("sha256"), str)
+            and len(snapshot["sha256"]) == 64
+            and 0 < snapshot.get("bytes", 0) <= nested.SNAPSHOT_MAX_BYTES
+            and snapshot.get("leaf_counts") == nested.LEAVES
+            and snapshot.get("step") == sample_steps(n)[0][-1]),
+    }
+
+
+def assess_nested(records, expected_revision=None, diagnostic=NESTED_SHEAR):
+    """Fixed four-case preset contract: nested_shear_n128 (U128, U512, constant/projected
+    R128 depth 3) or twin_shear_n128 (U128, U256, constant/projected R128 depth 2)."""
+    preset = PRESETS[diagnostic]
+    geometry = preset["module"]
+    if not isinstance(records, list) or not all(isinstance(r, dict) for r in records):
+        raise ValueError("records must be a list of receipt objects")
+    base_checks = tg.assess(records, expected_revision)["checks"]
+    checks = {name: base_checks.get(name, False) for name in CASE_AGNOSTIC}
+    indexed, duplicates = {}, []
+    for r in records:
+        key = tuple(r.get(k) for k in ("re_L", "n", "depth", "explosion"))
+        (duplicates.append(key) if key in indexed else indexed.__setitem__(key, r))
+    checks.update(complete_matrix=all(k in indexed for k in geometry.CASES),
+                  no_extra_cases=set(indexed) <= set(geometry.CASES),
+                  no_duplicate_cases=not duplicates,
+                  shear_diagnostic_identity=all(r.get("diagnostic") == diagnostic
+                                                for r in records))
+    for key in geometry.CASES:
+        _, n, depth, scheme = key
+        label = f"{n}/{depth}/{scheme}"
+        case = _nested_case_ok(indexed[key], n, depth, diagnostic) if key in indexed else {}
+        for name in ("complete_times", "scales", "actual_builds", "reference", "runtime_source",
+                     "patch", "workload", "shear_metadata", "nested_module_pin", "snapshot"):
+            checks[f"{label} {name}"] = bool(case.get(name, False))
+    contract_valid = all(checks.values())
+    result = dict(checks=checks, contract_valid=contract_valid, diagnostic=diagnostic,
+                  cases=[list(k) for k in geometry.CASES], expected_revision=expected_revision,
+                  finest_control=preset["finest_control"], scope=preset["scope"])
+    if not contract_valid:
+        result.update(status="INVALID_SET", passed=False)
+        return result
+    try:
+        cases = geometry.CASES
+        physics = preset["physics"](indexed[cases[0]], indexed[cases[1]],
+                                    [indexed[cases[2]], indexed[cases[3]]])
+    except (KeyError, ValueError) as error:
+        result.update(status="INVALID_SET", passed=False, reason=str(error))
+        return result
+    result.update(physics=physics, status=physics["status"], passed=physics["status"] == "PASS")
+    try:
+        # Role "uniform_fine" is the preset's finest control (4N nested, 2N twin).
+        result["signed"] = signed_summary([indexed[k] for k in geometry.CASES])
+    except (KeyError, TypeError, ValueError) as error:
+        result["signed_unavailable"] = str(error)
+    return result
+
+
 def _signed(record):
     """Raw signed quantities of one valid record (None when the fit is unavailable)."""
     s = sw.summarize(record)
@@ -167,11 +306,20 @@ if __name__ == "__main__":
     p.add_argument("files", type=Path, nargs=4)
     p.add_argument("--output", type=Path, required=True)
     p.add_argument("--expected-revision", required=True)
-    p.add_argument("--diagnostic", choices=[SCALED_SHEAR])
+    p.add_argument("--diagnostic", choices=[SCALED_SHEAR, NESTED_SHEAR, TWIN_SHEAR])
     p.add_argument("--reference-files", type=Path, nargs=4)
     args = p.parse_args()
     if args.output.exists():
         p.error("Output already exists; choose a new path to retain prior evidence")
+    if args.diagnostic in PRESETS:
+        if args.reference_files:
+            p.error(f"{args.diagnostic} takes no reference files (no resolution trend)")
+        result = assess_nested([json.loads(path.read_text()) for path in args.files],
+                               args.expected_revision, args.diagnostic)
+        text = json.dumps(result, indent=2, allow_nan=False) + "\n"
+        args.output.write_text(text)
+        print(text, end="")
+        raise SystemExit(0 if result["passed"] else 1)
     if bool(args.diagnostic) != bool(args.reference_files):
         p.error("--diagnostic scaled_shear_n256 requires --reference-files (N128 receipts)")
     result = assess([json.loads(path.read_text()) for path in args.files], args.expected_revision,

tests/validation/taylor_green/run_case.py

@@ -21,6 +21,11 @@ INITIALIZATION = "physical FP64 equilibrium G at actual cell centers; rho=1+3p"
 SHEAR_WAVE_SHA256 = "81d5c8ee60864372887cddf9239580c82fb6750d956038f1e3ee9de61aa4e877"
 SCALED_DIAGNOSTIC = "scaled_n256"
 SCALED_SHEAR = "scaled_shear_n256"  # mutually exclusive with scaled_n256 (one flow each)
+NESTED_SHEAR = "nested_shear_n128"  # fixed three-level/two-region shear preset
+SHEAR_NESTED_SHA256 = "fa6cfe87fb4df8c6d732c637c885276833c81973b950c7724d734f8fc350bcb0"
+TWIN_SHEAR = "twin_shear_n128"  # depth-2 twin: nested L0->L1 boxes without L2
+SHEAR_TWIN_SHA256 = "1cbf37f18e89aea577581631c4565f51e68d22a3739b698816ef64acbf2b2bb4"
+SHEAR_PROBES_SHA256 = "53ba8af8ac9c8078c40f84488c7ade4c11ccc91dd1fc275ff806042e9af30166"  # signed interface-probe mechanism diagnostic
 
 
 def flow_functions(flow="taylor_green"):
@@ -90,7 +95,7 @@ def provenance(repo):
     )
 
 
-def verify_source(repo, revision, flow="taylor_green"):
+def verify_source(repo, revision, flow="taylor_green", probes=False):
     """Exact clean harness commit, fixed production tree, and reviewed capture helper."""
     import oracle
     from tests.validation.multigrid import check_projected_pairs as gate
@@ -109,7 +114,28 @@ def verify_source(repo, revision, flow="taylor_green"):
     if result["source_tree_sha256"] != SOURCE_SHA256:
         raise RuntimeError("Wrong numerical source fingerprint")
     result["build_capture_sha256"] = BUILD_CAPTURE_SHA256
-    if flow == "shear_wave":
+    if probes:
+        import shear_probes
+        module = harness / "shear_probes.py"
+        if (Path(shear_probes.__file__).resolve() != module
+                or hashlib.sha256(module.read_bytes()).hexdigest() != SHEAR_PROBES_SHA256):
+            raise RuntimeError("Wrong shear_probes module checkout or content")
+        result["shear_probes_sha256"] = SHEAR_PROBES_SHA256
+    if flow == "shear_twin":
+        import shear_twin
+        module = harness / "shear_twin.py"
+        if (Path(shear_twin.__file__).resolve() != module
+                or hashlib.sha256(module.read_bytes()).hexdigest() != SHEAR_TWIN_SHA256):
+            raise RuntimeError("Wrong shear_twin module checkout or content")
+        result["shear_twin_sha256"] = SHEAR_TWIN_SHA256
+    if flow in ("shear_nested", "shear_twin"):  # the twin reuses the nested geometry
+        import shear_nested
+        module = harness / "shear_nested.py"
+        if (Path(shear_nested.__file__).resolve() != module
+                or hashlib.sha256(module.read_bytes()).hexdigest() != SHEAR_NESTED_SHA256):
+            raise RuntimeError("Wrong shear_nested module checkout or content")
+        result["shear_nested_sha256"] = SHEAR_NESTED_SHA256
+    if flow in ("shear_wave", "shear_nested", "shear_twin"):
         import shear_wave
         module = harness / "shear_wave.py"
         if (Path(shear_wave.__file__).resolve() != module
@@ -182,21 +208,50 @@ def main():
     p.add_argument("--depth", type=int, choices=[1, 2, 3], required=True)
     p.add_argument("--output", type=Path, required=True)
     p.add_argument("--explosion", choices=["constant", "projected_linear"], default="constant")
-    p.add_argument("--diagnostic", choices=[SCALED_DIAGNOSTIC, SCALED_SHEAR],
+    p.add_argument("--diagnostic",
+                   choices=[SCALED_DIAGNOSTIC, SCALED_SHEAR, NESTED_SHEAR, TWIN_SHEAR],
                    help="explicit four-case N256/fine512 same-placement diagnostic "
-                        "(scaled_n256: Taylor-Green; scaled_shear_n256: shear wave)")
+                        "(scaled_n256: Taylor-Green; scaled_shear_n256: shear wave) or the "
+                        "fixed three-level/two-region shear preset (nested_shear_n128) "
+                        "or its depth-2 twin (twin_shear_n128)")
+    p.add_argument("--probe-trace", action="store_true",
+                   help="signed interface-probe trace (nested U128/U512/projected d3, "
+                        "twin projected d2); mechanism diagnostic, no gates")
+    p.add_argument("--final-snapshot", action="store_true",
+                   help="nested_shear_n128 projected depth-3 only: final macro snapshot")
     p.add_argument("--early-pressure", action="store_true",
                    help="also save 65 scalar pressure samples at t*=k*.001875, k=0..64")
     p.add_argument("--flow", choices=["taylor_green", "shear_wave"], default="taylor_green")
     args = p.parse_args()
-    if args.flow == "shear_wave" and (args.re != 100 or args.depth == 3 or args.early_pressure):
+    nested = args.diagnostic == NESTED_SHEAR
+    if args.flow == "shear_wave" and (args.re != 100 or (args.depth == 3 and not nested)
+                                      or args.early_pressure):
         p.error("shear_wave requires --re 100, depth 1 or 2 and no --early-pressure")
+    probe_cases = {(NESTED_SHEAR, 128, 1, "constant"), (NESTED_SHEAR, 512, 1, "constant"),
+                   (NESTED_SHEAR, 128, 3, "projected_linear"),
+                   (TWIN_SHEAR, 128, 2, "projected_linear")}
+    if args.probe_trace and (args.final_snapshot or (
+            args.diagnostic, args.n, args.depth, args.explosion) not in probe_cases):
+        p.error("--probe-trace requires nested U128/U512/projected depth 3 or twin projected "
+                "depth 2, without --final-snapshot")
+    if args.final_snapshot and not (nested and args.depth == 3
+                                    and args.explosion == "projected_linear"):
+        p.error("--final-snapshot requires nested_shear_n128 projected depth 3")
     initial, metrics, limits, initialization = flow_functions(args.flow)
     if args.output.exists():
         p.error("Output already exists; choose a new path to retain prior evidence")
-    if args.explosion != "constant" and args.depth != 2:
+    if args.explosion != "constant" and args.depth != 2 and not nested:
         p.error("projected_linear requires depth 2")
-    if args.diagnostic:
+    twin = args.diagnostic == TWIN_SHEAR
+    if nested or twin:
+        import shear_nested
+        import shear_twin
+        preset = shear_nested if nested else shear_twin
+        if args.flow != "shear_wave":
+            p.error(f"--diagnostic {args.diagnostic} requires --flow shear_wave")
+        if (args.re, args.n, args.depth, args.explosion) not in preset.CASES:
+            p.error(f"{args.diagnostic} requires one of its four Re100 cases")
+    elif args.diagnostic:
         cases = {(256, 1, "constant"), (512, 1, "constant"),
                  (256, 2, "constant"), (256, 2, "projected_linear")}
         flow = "shear_wave" if args.diagnostic == SCALED_SHEAR else "taylor_green"
@@ -209,13 +264,15 @@ def main():
         if args.diagnostic == SCALED_SHEAR and (args.n, args.depth, args.explosion) not in cases:
             p.error("scaled_shear_n256 requires one of its four Re100 cases")
     elif args.n == 512:
-        p.error("N512 requires --diagnostic scaled_n256 or scaled_shear_n256")
+        p.error("N512 requires --diagnostic scaled_n256, scaled_shear_n256 or nested_shear_n128")
     selected = os.environ.get("CUDA_VISIBLE_DEVICES", "")
     if not args.dry_run and (not args.run_gpu or not selected or "," in selected or selected == "-1"):
         p.error("--run-gpu and exactly one explicit CUDA_VISIBLE_DEVICES are required")
     repo = args.repo.resolve()
     sys.path[:0] = [str(repo / "src"), str(repo)]
-    source = verify_source(repo, args.expected_revision, args.flow)
+    source = verify_source(repo, args.expected_revision,
+                           "shear_nested" if nested else "shear_twin" if twin else args.flow,
+                           probes=args.probe_trace)
     runtime = runtime_source(repo)
     if args.dry_run:
         print(json.dumps(dict(source=source, runtime_source=runtime, dry_run=True,
@@ -229,7 +286,12 @@ def main():
     from CelerisLab.lbm.hierarchy import build_hierarchy, leaf_partition
 
     cfg = config(args.n, args.re)
-    refinements = [] if args.depth == 1 else rectangles(args.n, args.depth)
+    if args.depth == 1:
+        refinements = []
+    elif nested or twin:
+        refinements = [list(level) for level in preset.REFINEMENTS]
+    else:
+        refinements = rectangles(args.n, args.depth)
     h = build_hierarchy(cfg, refinements)
     np.testing.assert_array_equal(leaf_partition(h), np.ones((args.n, args.n)))
     masks = [k == ACTIVE for k in h.kinds]
@@ -308,8 +370,23 @@ def main():
                 ]
             )
             initial_mass = None
-            for step in sorted(set(main_steps + early_steps)):
+            probe_steps = set()
+            if args.probe_trace:
+                import shear_probes
+                probe_root = dict(zip(shear_probes.case_steps(args.n), shear_probes.STEPS))
+                probe_steps = set(probe_root)
+                probes = shear_probes.select(masks, args.n)
+                report["probe_trace"] = shear_probes.new_trace(
+                    probes, cfg.viscosity, [args.diagnostic, args.n, args.depth, args.explosion])
+            for step in sorted(set(main_steps + early_steps) | probe_steps):
                 run.step(step - run.step_count)
+                if step in probe_steps:
+                    trace = report["probe_trace"]
+                    trace["steps"].append(probe_root[step])  # root-physical step
+                    trace["values"].append(shear_probes.sample(
+                        (run.download(),) if args.depth == 1 else run.download(), probes))
+                    if step not in main_steps and step not in early_steps:
+                        continue  # probe-only step: no qualification metrics
                 row = sample(run, masks, step, args.n, cfg.viscosity, initial_mass,
                              metrics, args.depth)
                 row["step"] = step
@@ -317,6 +394,13 @@ def main():
                     initial_mass = row["mass"]
                 if step in main_steps:
                     report["rows"].append(row)
+                if args.final_snapshot and step == main_steps[-1]:
+                    # Optional macro-only snapshot; scalar receipt stays authoritative.
+                    report["final_snapshot"] = shear_nested.write_snapshot(
+                        args.output.with_name(args.output.stem + ".final_macro.npz"),
+                        run.download(), masks,
+                        shear_nested.snapshot_meta(step, cfg.viscosity, args.explosion,
+                                                   source.get("revision")))
                 if step in early_steps:
                     trace = report["early_pressure"]
                     trace["rows"].append({key: row[key] for key in ("step", "tstar", "pressure_l2")})
@@ -355,6 +439,11 @@ def main():
                         ),
                         flush=True,
                     )
+    if args.probe_trace:
+        try:
+            report["probe_trace"]["bytes"] = shear_probes.validate(report["probe_trace"])
+        except ValueError as error:  # keep the scalar receipt; drop an invalid trace
+            report["probe_trace"] = dict(error=str(error))
     report.update(
         completed=True,
         seconds=time.monotonic() - started,

tests/validation/taylor_green/shear_nested.py

@@ -0,0 +1,174 @@
+"""Fixed three-level/two-region shear preset (opt-in nested_shear_n128).
+
+Four fresh cases: U128, U512 (finest control = 4N), constant R128 depth 3,
+projected R128 depth 3. Same wave, phase, Re100/U=0.03, initialization, five
+sample times, eps(root N), limits and floors as shear_wave. This changes
+placement and refined volume as well as depth: no isolated-level cost or
+resolution-trend label. shear_wave.py is reused unchanged (historical receipt
+identities stay valid); only the depth/finest-control contract is generalized
+here.
+"""
+
+import hashlib
+import json
+from pathlib import Path
+
+import numpy as np
+
+import shear_wave as sw
+from oracle import CX, CY, U, centers
+
+ROOT_N = 128
+DEPTH = 3
+FINEST_FACTOR = 2 ** (DEPTH - 1)  # finest-spacing uniform control is 4N
+# Parent-frame (x0, y0, x1, y1) rectangles; level-wide, zero-origin unions.
+REFINEMENTS = [
+    [(16, 16, 60, 112), (68, 24, 112, 120)],     # L0 -> L1, root coordinates
+    [(44, 44, 108, 212), (148, 60, 212, 228)],   # L1 -> L2, L1 coordinates
+]
+CASES = ((100, ROOT_N, 1, "constant"), (100, FINEST_FACTOR * ROOT_N, 1, "constant"),
+         (100, ROOT_N, DEPTH, "constant"), (100, ROOT_N, DEPTH, "projected_linear"))
+ROOT_STEPS = 16 * ROOT_N
+LEAVES = [7936, 12288, 86016]
+HOPS = [ROOT_STEPS * 2**level for level in range(DEPTH)]  # [2048, 4096, 8192]
+WORK = sum(c * h for c, h in zip(LEAVES, HOPS))           # 771227648 per refined case
+NU = [U * ROOT_N / sw.RE * 2**level for level in range(DEPTH)]  # [.0384, .0768, .1536]
+TAU = [0.5 + 3 * nu for nu in NU]                                # [.6152, .7304, .9608]
+SNAPSHOT_MAX_BYTES = 8 * 1024 * 1024
+SNAPSHOT_KEYS = ("level", "i", "j", "rho", "ux", "uy", "meta")
+
+
+def leaf_masks():
+    """Independent numpy leaf masks of the fixed preset (root, L1, L2 level-wide grids)."""
+    masks, covered = [], None
+    for level in range(DEPTH):
+        size = ROOT_N * 2**level
+        inside = np.ones((size, size), bool) if level == 0 else covered
+        if level < DEPTH - 1:
+            child = np.zeros((size, size), bool)
+            for x0, y0, x1, y1 in REFINEMENTS[level]:
+                child[y0:y1, x0:x1] = True
+            masks.append(inside & ~child)
+            covered = np.kron(child, np.ones((2, 2), bool))
+        else:
+            masks.append(inside)
+    return masks
+
+
+def assess(base_control, finest_control, refined):
+    """shear_wave.assess generalized to depth 3 with a 4N finest control.
+
+    Same summaries, screens, budgets and outcome semantics; only the
+    (depth, finest-control) shape contract differs.
+    """
+    if base_control is None or finest_control is None or not refined:
+        return dict(status="INVALID_CONTROL", reason="missing control or refined run")
+    n = base_control["n"]
+    if (base_control["depth"], finest_control["depth"], finest_control["n"]) != (
+            1, 1, FINEST_FACTOR * n):
+        raise ValueError("controls must be uniform N and uniform 4N")
+    if any(r["depth"] != DEPTH or r["n"] != n for r in refined):
+        raise ValueError("refined runs must be depth 3 at root N")
+    if len({r.get("explosion") for r in refined}) != len(refined):
+        raise ValueError("duplicate refined explosion scheme")
+    sources = {repr(r.get("source")) for r in [base_control, finest_control, *refined]}
+    base, fine = sw._usable(base_control), sw._usable(finest_control)
+    if base is None or fine is None or len(sources) != 1:
+        return dict(status="INVALID_CONTROL", reason="incomplete/invalid control or mixed source")
+    controls_pass = base["own_screens_pass"] and fine["own_screens_pass"]
+    report = dict(base_control=base, finest_control=fine, refined={},
+                  controls_pass=controls_pass)
+    statuses = []
+    for r in refined:
+        try:
+            s = sw.summarize(r)
+        except (KeyError, TypeError, ValueError) as error:
+            report["refined"][r["explosion"]] = dict(valid=False, reason=str(error))
+            statuses.append("INVALID_RUN")
+            continue
+        s["valid"] = True
+        s["budget"] = {
+            key: (None if base[key] is None else max(sw.BASELINE_FACTOR * base[key], floor))
+            for key, floor in sw.FLOORS.items()
+        }
+        s["budget_pass"] = all(
+            s[key] is not None and s["budget"][key] is not None and s[key] <= s["budget"][key]
+            for key in sw.FLOORS)
+        s["control_ratios"] = {
+            name: {key: (s[key] / c[key] if s[key] is not None and c[key] else None)
+                   for key in (*sw.FLOORS, "spurious_pressure_rms")}
+            for name, c in (("root", base), ("finest", fine))
+        }
+        s["controlled_pass"] = controls_pass and s["own_screens_pass"] and s["budget_pass"]
+        report["refined"][r["explosion"]] = s
+        statuses.append("PASS" if s["controlled_pass"] else "FAIL")
+    if "INVALID_RUN" in statuses:
+        report["status"] = "INVALID_RUN"
+    elif not controls_pass:
+        report["status"] = "CONTROL_SCREEN_FAIL"
+    else:
+        report["status"] = "PASS" if all(v == "PASS" for v in statuses) else "FAIL"
+    return report
+
+
+def write_snapshot(path, arrays, masks, meta):
+    """Final macro snapshot: packed ACTIVE leaves (level, i, j), rho, ux, uy only.
+
+    No populations, no restart state, no time series. Returns the receipt block;
+    a file over SNAPSHOT_MAX_BYTES is removed and reported, never kept.
+    """
+    path = Path(path)
+    if path.exists():
+        raise FileExistsError(path)
+    cols = {k: [] for k in ("level", "i", "j", "rho", "ux", "uy")}
+    counts = []
+    for level, (g, mask) in enumerate(zip(arrays, masks)):
+        if g.shape != (9, *mask.shape):
+            raise ValueError("population/mask shape mismatch")
+        values = g[:, mask].astype(float)
+        rho = values.sum(axis=0)
+        j, i = np.nonzero(mask)  # row = y index, column = x index
+        cols["level"].append(np.full(rho.size, level, np.uint8))
+        cols["i"].append(i.astype(np.uint16))
+        cols["j"].append(j.astype(np.uint16))
+        cols["rho"].append(rho)
+        cols["ux"].append(CX @ values / rho)
+        cols["uy"].append(CY @ values / rho)
+        counts.append(int(rho.size))
+    packed = {k: np.concatenate(v) for k, v in cols.items()}
+    if not all(np.isfinite(packed[k]).all() for k in ("rho", "ux", "uy")):
+        raise FloatingPointError("nonfinite snapshot macros")
+    meta = dict(meta, leaf_counts=counts, cell_center="h*(index+0.5), h=2**-level",
+                fields="rho, ux, uy from FP64 physical populations at ACTIVE leaves")
+    with path.open("xb") as handle:
+        np.savez_compressed(handle, meta=np.array(json.dumps(meta, sort_keys=True)), **packed)
+    size = path.stat().st_size
+    block = dict(file=path.name, bytes=size, leaf_counts=counts, max_bytes=SNAPSHOT_MAX_BYTES,
+                 step=meta.get("step"), tstar=meta.get("tstar"))
+    if size > SNAPSHOT_MAX_BYTES:
+        path.unlink()
+        return dict(block, kept=False, sha256=None, reason="exceeds snapshot byte cap")
+    block.update(kept=True, sha256=hashlib.sha256(path.read_bytes()).hexdigest())
+    return block
+
+
+def read_snapshot(path):
+    """Validate a snapshot: exact keys, no populations, consistent packed arrays."""
+    with np.load(path, allow_pickle=False) as data:
+        if tuple(sorted(data.files)) != tuple(sorted(SNAPSHOT_KEYS)):
+            raise ValueError("unexpected snapshot contents")
+        out = {k: data[k] for k in data.files}
+    meta = json.loads(str(out.pop("meta")))
+    n = len(out["rho"])
+    if any(len(v) != n for v in out.values()) or sum(meta["leaf_counts"]) != n:
+        raise ValueError("inconsistent snapshot arrays")
+    return meta, out
+
+
+def snapshot_meta(step, viscosity, explosion, source_revision):
+    """Exact time/phase/config identity stored inside the snapshot."""
+    return dict(step=step, tstar=step * U / ROOT_N, phase=sw.PHI, mode=list(sw.MODE),
+                polarization=sw.E.tolist(), amplitude=U, re_L=sw.RE, n=ROOT_N, depth=DEPTH,
+                explosion=explosion, viscosity=viscosity, tau_by_level=TAU,
+                refinements=REFINEMENTS, source_revision=source_revision,
+                origin="level-wide zero-origin grids", time="root synchronization, postcollision")

tests/validation/taylor_green/shear_probes.py

@@ -0,0 +1,123 @@
+"""Signed interface-probe traces for the nested/twin shear presets (mechanism diagnostic).
+
+No pass gates. Native-validated 24-target layout through box 1, in root-128
+physical coordinates, crossing L0->L1 and (nested only) L1->L2 away from corners:
+  horizontal line y = 56 across the vertical faces x = 16 and x = 22;
+  vertical line x = 38 across the horizontal faces y = 16 and y = 22;
+  normal coordinates NORMAL_COORDS on both (nested: 3 L0 + 5 L1 + 4 L2 each).
+Each target maps to the nearest ACTIVE leaf center (tie rule: distance, then
+finer level, then smaller j, then smaller i). Grids differ between cases
+(U128, U512, nested, twin), so actual centers, levels and displacements are
+stored and analytic fields are evaluated at the actual centers.
+Samples are POST-collision G at root synchronization only (root-step units);
+no substep content is observed, and grid-scale alternation can alias.
+"""
+
+import math
+
+import numpy as np
+
+import shear_wave as sw
+from oracle import CX, CY, U
+
+ROOT_N = 128
+NORMAL_COORDS = (12.5, 14.5, 15.5, 16.25, 17.25, 19.25, 20.75, 21.75,
+                 22.125, 23.125, 24.125, 26.125)
+TRANSECTS = {"horizontal_y56": dict(axis=0, other=56.0),   # x varies, y fixed
+             "vertical_x38": dict(axis=1, other=38.0)}     # y varies, x fixed
+STEPS = tuple(range(0, 129)) + tuple(range(1920, 2049))  # root steps, both inclusive
+TRACE_MAX_BYTES = 600_000
+
+
+def points():
+    out = []
+    for name, t in TRANSECTS.items():
+        for along in NORMAL_COORDS:
+            xy = (along, t["other"]) if t["axis"] == 0 else (t["other"], along)
+            out.append((name, along, xy))
+    return out
+
+
+def case_steps(n):
+    """Case lattice steps at the fixed root-physical sample times (U512: 4 per root step)."""
+    return [s * (n // ROOT_N) for s in STEPS]
+
+
+def select(masks, n=ROOT_N):
+    """Nearest ACTIVE leaf center for each target (deterministic tie rule).
+
+    Targets and stored centers are in root-physical units (root cell = 1); a case
+    with root N = n has cell size h = ROOT_N / (n * 2**level) there (U512: 0.25).
+    """
+    if n % ROOT_N:
+        raise ValueError("case root N must be a multiple of 128")
+    probes = []
+    for name, along, (x, y) in points():
+        best = None
+        for level, mask in enumerate(masks):
+            h = ROOT_N / (n * 2 ** level)
+            i0, j0 = int(math.floor(x / h)), int(math.floor(y / h))
+            for j in range(j0 - 2, j0 + 3):
+                for i in range(i0 - 2, i0 + 3):
+                    if 0 <= j < mask.shape[0] and 0 <= i < mask.shape[1] and mask[j, i]:
+                        cx, cy = h * (i + 0.5), h * (j + 0.5)
+                        key = (math.hypot(cx - x, cy - y), -level, j, i)
+                        if best is None or key < best[0]:
+                            best = (key, level, i, j, cx, cy)
+        if best is None:
+            raise ValueError(f"no ACTIVE leaf near target {(x, y)}")
+        _, level, i, j, cx, cy = best
+        probes.append(dict(transect=name, normal=along, point=[x, y], level=level, i=i, j=j,
+                           center=[cx, cy], displacement=[cx - x, cy - y]))
+    if len({(p["level"], p["i"], p["j"]) for p in probes}) != len(probes):
+        raise ValueError("probe targets map to duplicate cells")
+    return probes
+
+
+def sample(arrays, probes):
+    """Flat [rho, ux, uy] per probe from FP64 physical populations."""
+    row = []
+    for p in probes:
+        f = np.asarray(arrays[p["level"]][:, p["j"], p["i"]], dtype=float)
+        rho = float(f.sum())
+        row += [rho, float(CX @ f) / rho, float(CY @ f) / rho]
+    return row
+
+
+def new_trace(probes, case_viscosity, case):
+    """Times/coordinates are root-physical (N=128); nu_root = U*128/Re for all cases."""
+    return dict(schema=1, kind="mechanism diagnostic, no pass gates", case=case,
+                time_unit="root step (N=128)", sample_phase="postcollision G at root synchronization",
+                root_n=ROOT_N, viscosity_root=U * ROOT_N / sw.RE, case_viscosity=case_viscosity,
+                mode=list(sw.MODE), phase=sw.PHI,
+                amplitude=U, fields=["rho", "ux", "uy"], probes=probes, steps=[], values=[])
+
+
+def residuals(trace):
+    """Signed (drho, dux, duy) per step and probe, analytic field at each actual center."""
+    out = []
+    for step, row in zip(trace["steps"], trace["values"]):
+        res = []
+        for n, p in enumerate(trace["probes"]):
+            x, y = p["center"]
+            ux, uy, _ = sw.exact(x, y, step, trace["root_n"], trace["viscosity_root"])
+            rho, vx, vy = row[3 * n: 3 * n + 3]
+            res.append((rho - 1.0, vx - float(ux), vy - float(uy)))
+        out.append(res)
+    return out
+
+
+def validate(trace):
+    """Exact schedule, finite values, fixed probe count and byte cap."""
+    import json
+    if trace.get("steps") != list(STEPS) or len(trace["values"]) != len(STEPS):
+        raise ValueError("trace steps differ from the fixed schedule")
+    if len(trace["probes"]) != len(NORMAL_COORDS) * len(TRANSECTS):
+        raise ValueError("unexpected probe count")
+    width = 3 * len(trace["probes"])
+    if any(len(r) != width or not all(math.isfinite(v) for v in r) for r in trace["values"]):
+        raise ValueError("malformed or nonfinite trace row")
+    size = len(json.dumps(trace, allow_nan=False).encode())
+    if size > TRACE_MAX_BYTES:
+        raise ValueError(f"trace exceeds {TRACE_MAX_BYTES} bytes ({size})")
+    return size

tests/validation/taylor_green/shear_twin.py

@@ -0,0 +1,36 @@
+"""Depth-2 twin of the nested shear preset (opt-in twin_shear_n128).
+
+Same two L0->L1 boxes as shear_nested, no L2. Four fresh cases: U128, U256
+(finest control = 2N), constant R128 depth 2, projected R128 depth 2; same
+wave, phase, Re100/U=0.03, initialization, times, eps(root N), limits, floors.
+
+Twin vs nested measures the effect of adding this nested L2 refinement.
+Removing L2 also changes the finest spacing and refined volume, so the
+contrast does not isolate abstract depth. Physics is the unchanged depth-2,
+2N-control shear_wave.assess; shear_nested.py stays byte-identical.
+"""
+
+import numpy as np
+
+import shear_nested as nested
+
+ROOT_N = nested.ROOT_N
+DEPTH = 2
+FINEST_FACTOR = 2
+REFINEMENTS = [nested.REFINEMENTS[0]]  # L0 -> L1 only, root coordinates
+CASES = ((100, ROOT_N, 1, "constant"), (100, FINEST_FACTOR * ROOT_N, 1, "constant"),
+         (100, ROOT_N, DEPTH, "constant"), (100, ROOT_N, DEPTH, "projected_linear"))
+ROOT_STEPS = 16 * ROOT_N
+LEAVES = [7936, 33792]
+HOPS = [ROOT_STEPS * 2**level for level in range(DEPTH)]  # [2048, 4096]
+WORK = sum(c * h for c, h in zip(LEAVES, HOPS))           # 154664960 per refined case
+NU = nested.NU[:DEPTH]
+TAU = nested.TAU[:DEPTH]
+
+
+def leaf_masks():
+    """Independent numpy leaf masks (root complement, full L1 boxes)."""
+    child = np.zeros((ROOT_N, ROOT_N), bool)
+    for x0, y0, x1, y1 in REFINEMENTS[0]:
+        child[y0:y1, x0:x1] = True
+    return [~child, np.kron(child, np.ones((2, 2), bool))]

tests/validation/taylor_green/test_shear_wave.py

@@ -427,6 +427,19 @@ class ContractTests(unittest.TestCase):
         self.assertTrue(result["contract_valid"], [k for k, v in result["checks"].items() if not v])
         self.assertEqual(result["status"], "PASS")
 
+    def test_eps_metadata_is_portable_but_bounded(self):
+        import assess_shear_wave
+        self.assertTrue(assess_shear_wave.close_ulps(0.012047856934923534, 0.012047856934923532))
+        exact = sw.eps(128)
+        for value, ok in ((math.nextafter(exact, 1.0), True),
+                          (math.nextafter(math.nextafter(exact, 0.0), 0.0), True),
+                          (exact * (1 + 1e-12), False), (exact * 2, False),
+                          (str(exact), False), (float("nan"), False), (None, False)):
+            records = [dict(r) for r in self.records]
+            records[2]["eps"] = value
+            status = self.assess(records, "r")["status"]
+            self.assertEqual(status == "PASS", ok, (value, status))
+
     def test_reproduced_false_passes_are_rejected(self):
         a = self.assess
         self.assertEqual(a(self.records[:2], "r")["status"], "INVALID_SET")  # empty refined
@@ -558,3 +571,554 @@ class DefaultPathTests(unittest.TestCase):
 
 if __name__ == "__main__":
     unittest.main()
+
+
+class NestedPresetTests(unittest.TestCase):
+    """Fixed three-level/two-region shear preset (nested_shear_n128)."""
+
+    def setUp(self):
+        import assess_shear_wave
+        import shear_nested
+        self.mod, self.nested = assess_shear_wave, shear_nested
+
+    def test_exact_geometry_work_and_levels(self):
+        nested = self.nested
+        masks = nested.leaf_masks()
+        self.assertEqual([int(m.sum()) for m in masks], [7936, 12288, 86016])
+        self.assertEqual(nested.LEAVES, [7936, 12288, 86016])
+        self.assertEqual(nested.HOPS, [2048, 4096, 8192])
+        self.assertEqual(nested.WORK, 771227648)
+        uniform = 128 * 128 * 2048 + 512 * 512 * 8192
+        self.assertEqual(uniform + 2 * nested.WORK, 3723493376)
+        np.testing.assert_allclose(nested.NU, [.0384, .0768, .1536], rtol=0, atol=1e-15)
+        np.testing.assert_allclose(nested.TAU, [.6152, .7304, .9608], rtol=0, atol=1e-14)
+        self.assertEqual(sum(float(m.sum()) * 0.25**l for l, m in enumerate(masks)), 128.0**2)
+        # Production host hierarchy builds the same leaves from the same rectangles.
+        from CelerisLab.lbm.hierarchy import build_hierarchy, leaf_partition
+        from CelerisLab.lbm.refinement import ACTIVE
+        h = build_hierarchy(run_case.config(128, 100), nested.REFINEMENTS)
+        np.testing.assert_array_equal(leaf_partition(h), np.ones((128, 128)))
+        for mine, kinds in zip(masks, h.kinds):
+            np.testing.assert_array_equal(mine, kinds == ACTIVE)
+
+    def test_historical_shear_identity_is_preserved(self):
+        digest = hashlib.sha256((HERE / "shear_wave.py").read_bytes()).hexdigest()
+        self.assertEqual(digest, "81d5c8ee60864372887cddf9239580c82fb6750d956038f1e3ee9de61aa4e877")
+        self.assertEqual(run_case.SHEAR_WAVE_SHA256, digest)
+        nested_digest = hashlib.sha256((HERE / "shear_nested.py").read_bytes()).hexdigest()
+        self.assertEqual(run_case.SHEAR_NESTED_SHA256, nested_digest)
+
+    def nested_set(self):
+        out = []
+        for n, depth, scheme in ((128, 1, "constant"), (512, 1, "constant"),
+                                 (128, 3, "constant"), (128, 3, "projected_linear")):
+            r = receipt(n, depth, scheme, diagnostic=run_case.NESTED_SHEAR)
+            r["source"]["shear_nested_sha256"] = run_case.SHEAR_NESTED_SHA256
+            if depth == 3:
+                cells, hops = self.nested.LEAVES, self.nested.HOPS
+                r["refinements"] = [list(level) for level in self.nested.REFINEMENTS]
+                r["workload"] = dict(active_cells_by_level=cells, level_hops=hops,
+                                     root_intervals=2048,
+                                     active_collision_updates_by_level=[c * h for c, h in zip(cells, hops)],
+                                     active_collision_updates=self.nested.WORK)
+            out.append(r)
+        return out
+
+    def test_nested_contract_and_rejections(self):
+        a = self.mod.assess_nested
+        good = self.nested_set()
+        result = a(good, "r")
+        self.assertTrue(result["contract_valid"], [k for k, v in result["checks"].items() if not v])
+        self.assertEqual(result["status"], "PASS")
+        self.assertIn("finest", result["physics"]["refined"]["constant"]["control_ratios"])
+        wrong_finest = self.nested_set()
+        wrong_finest[1] = receipt(256, 1, diagnostic=run_case.NESTED_SHEAR)
+        wrong_finest[1]["source"]["shear_nested_sha256"] = run_case.SHEAR_NESTED_SHA256
+        self.assertEqual(a(wrong_finest, "r")["status"], "INVALID_SET")
+        for mutate in (
+            lambda s: s[2].update(refinements=[s[2]["refinements"][1], s[2]["refinements"][0]]),
+            lambda s: s[3].update(refinements=rectangles(128, 3)),
+            lambda s: s[2]["rows"][2].update(spurious_pressure_scaled_by_level=[0.0, 0.0]),
+            lambda s: s[0].update(diagnostic=run_case.SCALED_SHEAR),
+            lambda s: s[2]["source"].pop("shear_nested_sha256"),
+            lambda s: s[2].update(final_snapshot=dict(kept=True, sha256="a" * 64, bytes=10,
+                                                      leaf_counts=self.nested.LEAVES, step=2048)),
+            lambda s: s[3].update(final_snapshot=dict(kept=True, sha256="a" * 64,
+                                                      bytes=self.nested.SNAPSHOT_MAX_BYTES + 1,
+                                                      leaf_counts=self.nested.LEAVES, step=2048)),
+        ):
+            records = self.nested_set()
+            mutate(records)
+            self.assertNotEqual(a(records, "r")["status"], "PASS")
+        ok = self.nested_set()
+        ok[3]["final_snapshot"] = dict(kept=True, sha256="a" * 64, bytes=1000,
+                                       leaf_counts=self.nested.LEAVES, step=2048)
+        self.assertEqual(a(ok, "r")["status"], "PASS")
+        with self.assertRaises(ValueError):
+            self.nested.assess(good[0], good[1], [good[2], dict(good[2])])
+
+    def test_nested_runner_options(self):
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            base = ["run_case.py", "--repo", str(repo), "--expected-revision", "f" * 40,
+                    "--output", str(Path(tmp) / "case.json"), "--dry-run", "--re", "100"]
+            nest = ["--flow", "shear_wave", "--diagnostic", run_case.NESTED_SHEAR]
+            proj = ["--explosion", "projected_linear"]
+            for extra, valid in (
+                    (["--n", "128", "--depth", "1"] + nest, True),
+                    (["--n", "512", "--depth", "1"] + nest, True),
+                    (["--n", "128", "--depth", "3"] + nest, True),
+                    (["--n", "128", "--depth", "3"] + proj + nest + ["--final-snapshot"], True),
+                    (["--n", "256", "--depth", "1"] + nest, False),      # finest control is 4N
+                    (["--n", "128", "--depth", "2"] + nest, False),
+                    (["--n", "128", "--depth", "3", "--flow", "shear_wave"], False),
+                    (["--n", "128", "--depth", "3", "--diagnostic", run_case.NESTED_SHEAR], False),
+                    (["--n", "128", "--depth", "3"] + nest + ["--final-snapshot"], False),
+                    (["--n", "128", "--depth", "3"] + proj + ["--flow", "shear_wave"], False),
+                    (["--n", "128", "--depth", "2", "--flow", "shear_wave", "--final-snapshot"], False)):
+                argv = base + extra
+                with self.subTest(argv=argv), patch.object(sys, "argv", argv), \
+                     patch.object(run_case, "verify_source", return_value={}) as verify, \
+                     patch.object(run_case, "runtime_source", return_value={}), \
+                     redirect_stdout(io.StringIO()), patch("sys.stderr", io.StringIO()):
+                    if valid:
+                        self.assertEqual(run_case.main(), 0)
+                        self.assertEqual(verify.call_args.args[2], "shear_nested")
+                    else:
+                        with self.assertRaises(SystemExit):
+                            run_case.main()
+
+    def test_mocked_nested_execution_with_bounded_snapshot(self):
+        from types import SimpleNamespace
+        from contextlib import nullcontext
+        from CelerisLab.lbm.hierarchy import build_hierarchy
+        h = build_hierarchy(run_case.config(128, 100), self.nested.REFINEMENTS)
+        nu = U * 128 / sw.RE
+        arrays = [sw.initial(k.shape, level, 128, nu) for level, k in enumerate(h.kinds)]
+
+        class FakeRun:
+            def __init__(self, *args, **kwargs):
+                self.step_count = 0
+                self.kwargs = kwargs
+
+            def __enter__(self):
+                return self
+
+            def __exit__(self, *args):
+                pass
+
+            def initialize(self, *given):
+                assert len(given) == 3
+
+            def step(self, count):
+                assert count >= 0
+                self.step_count += count
+
+            def download(self):
+                return arrays
+
+        modules = {"CelerisLab.cuda.context": SimpleNamespace(CudaContext=FakeRun),
+                   "CelerisLab.lbm.block_stepper": SimpleNamespace(PeriodicBlockStepper=FakeRun),
+                   "CelerisLab.lbm.refinement_stepper": SimpleNamespace(FixedPatchStepper=FakeRun)}
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            output = Path(tmp) / "projected_nested.json"
+            argv = ["run_case.py", "--repo", str(repo), "--re", "100", "--n", "128", "--depth", "3",
+                    "--explosion", "projected_linear", "--flow", "shear_wave",
+                    "--diagnostic", run_case.NESTED_SHEAR, "--final-snapshot",
+                    "--output", str(output), "--expected-revision", "f" * 40, "--run-gpu"]
+            with patch.dict(sys.modules, modules), patch.object(sys, "argv", argv), \
+                 patch.dict("os.environ", CUDA_VISIBLE_DEVICES="mock"), \
+                 patch.object(run_case, "verify_source", return_value=dict(revision="f" * 40)), \
+                 patch.object(run_case, "runtime_source", return_value={}), \
+                 patch.object(run_case, "runtime_identity", return_value={}), \
+                 patch.object(run_case, "build_capture", return_value=nullcontext()), \
+                 redirect_stdout(io.StringIO()):
+                run_case.main()
+            rec = json.loads(output.read_text())
+            self.assertTrue(rec["completed"])
+            self.assertEqual(rec["diagnostic"], run_case.NESTED_SHEAR)
+            self.assertEqual(rec["workload"]["active_cells_by_level"], self.nested.LEAVES)
+            self.assertEqual(rec["workload"]["active_collision_updates"], self.nested.WORK)
+            self.assertEqual(rec["tau_by_level"], self.nested.TAU)
+            self.assertTrue(all(len(r["spurious_pressure_scaled_by_level"]) == 3 for r in rec["rows"]))
+            snap = rec["final_snapshot"]
+            path = output.with_name(output.stem + ".final_macro.npz")
+            self.assertTrue(snap["kept"])
+            self.assertEqual(snap["sha256"], hashlib.sha256(path.read_bytes()).hexdigest())
+            self.assertLessEqual(snap["bytes"], self.nested.SNAPSHOT_MAX_BYTES)
+            meta, fields = self.nested.read_snapshot(path)
+            self.assertEqual((meta["step"], meta["phase"]), (2048, sw.PHI))
+            self.assertEqual(meta["tstar"], 2048 * U / 128)
+            self.assertEqual(meta["leaf_counts"], self.nested.LEAVES)
+            self.assertEqual(sorted(fields), ["i", "j", "level", "rho", "ux", "uy"])
+            np.testing.assert_allclose(fields["rho"], 1.0, atol=1e-12)
+
+    def test_snapshot_contents_and_cap(self):
+        nested = self.nested
+        masks = nested.leaf_masks()
+        arrays = [sw.initial(m.shape, level, 128, 0.0384) for level, m in enumerate(masks)]
+        meta = nested.snapshot_meta(2048, 0.0384, "projected_linear", "r")
+        with tempfile.TemporaryDirectory() as tmp:
+            path = Path(tmp) / "s.npz"
+            block = nested.write_snapshot(path, arrays, masks, meta)
+            self.assertTrue(block["kept"])
+            got, fields = nested.read_snapshot(path)
+            self.assertEqual(got["refinements"], [[list(r) for r in level] for level in nested.REFINEMENTS])
+            self.assertEqual(int(fields["level"].max()), 2)
+            with self.assertRaises(FileExistsError):
+                nested.write_snapshot(path, arrays, masks, meta)
+            bad = Path(tmp) / "bad.npz"
+            np.savez_compressed(bad, populations=np.zeros(3), **{k: np.zeros(1) for k in ("level", "i", "j", "rho", "ux", "uy")}, meta=np.array("{}"))
+            with self.assertRaises(ValueError):
+                nested.read_snapshot(bad)
+            with patch.object(nested, "SNAPSHOT_MAX_BYTES", 10):
+                capped = nested.write_snapshot(Path(tmp) / "c.npz", arrays, masks, meta)
+            self.assertFalse(capped["kept"])
+            self.assertFalse((Path(tmp) / "c.npz").exists())
+
+
+class TwinPresetTests(unittest.TestCase):
+    """Depth-2 twin (twin_shear_n128): nested L0->L1 boxes, no L2, fresh U128/U256."""
+
+    def setUp(self):
+        import assess_shear_wave
+        import shear_nested
+        import shear_twin
+        self.mod, self.nested, self.twin = assess_shear_wave, shear_nested, shear_twin
+
+    def test_exact_geometry_work_and_nested_identity(self):
+        twin = self.twin
+        masks = twin.leaf_masks()
+        self.assertEqual([int(m.sum()) for m in masks], [7936, 33792])
+        self.assertEqual((twin.LEAVES, twin.HOPS, twin.WORK), ([7936, 33792], [2048, 4096], 154664960))
+        self.assertEqual(128 * 128 * 2048 + 256 * 256 * 4096 + 2 * twin.WORK, 611319808)
+        self.assertEqual(twin.REFINEMENTS, [self.nested.REFINEMENTS[0]])
+        np.testing.assert_array_equal(masks[0], self.nested.leaf_masks()[0])  # same root leaves
+        from CelerisLab.lbm.hierarchy import build_hierarchy, leaf_partition
+        from CelerisLab.lbm.refinement import ACTIVE
+        h = build_hierarchy(run_case.config(128, 100), twin.REFINEMENTS)
+        np.testing.assert_array_equal(leaf_partition(h), np.ones((128, 128)))
+        for mine, kinds in zip(masks, h.kinds):
+            np.testing.assert_array_equal(mine, kinds == ACTIVE)
+        # Historical nested identity is unchanged; the twin is pinned separately.
+        self.assertEqual(hashlib.sha256((HERE / "shear_nested.py").read_bytes()).hexdigest(),
+                         "fa6cfe87fb4df8c6d732c637c885276833c81973b950c7724d734f8fc350bcb0")
+        self.assertEqual(run_case.SHEAR_TWIN_SHA256,
+                         hashlib.sha256((HERE / "shear_twin.py").read_bytes()).hexdigest())
+
+    def twin_set(self):
+        out = []
+        for n, depth, scheme in ((128, 1, "constant"), (256, 1, "constant"),
+                                 (128, 2, "constant"), (128, 2, "projected_linear")):
+            r = receipt(n, depth, scheme, diagnostic=run_case.TWIN_SHEAR)
+            r["source"].update(shear_nested_sha256=run_case.SHEAR_NESTED_SHA256,
+                               shear_twin_sha256=run_case.SHEAR_TWIN_SHA256)
+            if depth == 2:
+                cells, hops = self.twin.LEAVES, self.twin.HOPS
+                r["refinements"] = [list(level) for level in self.twin.REFINEMENTS]
+                r["workload"] = dict(active_cells_by_level=cells, level_hops=hops, root_intervals=2048,
+                                     active_collision_updates_by_level=[c * h for c, h in zip(cells, hops)],
+                                     active_collision_updates=self.twin.WORK)
+            out.append(r)
+        return out
+
+    def test_twin_contract_and_rejections(self):
+        a = lambda records: self.mod.assess_nested(records, "r", run_case.TWIN_SHEAR)
+        result = a(self.twin_set())
+        self.assertTrue(result["contract_valid"], [k for k, v in result["checks"].items() if not v])
+        self.assertEqual(result["status"], "PASS")
+        self.assertIn("does not isolate abstract depth", result["scope"])
+        self.assertEqual(result["finest_control"], "uniform 2N (U256)")
+        for mutate in (
+            lambda s: s.__setitem__(1, receipt(512, 1, diagnostic=run_case.TWIN_SHEAR)),  # 4N control
+            lambda s: s[2].update(refinements=[list(level) for level in self.nested.REFINEMENTS]),
+            lambda s: s[3].update(refinements=rectangles(128, 2)),  # old single box
+            lambda s: s[2]["rows"][1].update(spurious_pressure_scaled_by_level=[0.0, 0.0, 0.0]),
+            lambda s: s[0].update(diagnostic=run_case.NESTED_SHEAR),
+            lambda s: s[2]["source"].pop("shear_twin_sha256"),
+            lambda s: s[3].update(final_snapshot=dict(kept=True, sha256="a" * 64, bytes=10,
+                                                      leaf_counts=self.twin.LEAVES, step=2048)),
+        ):
+            records = self.twin_set()
+            mutate(records)
+            self.assertNotEqual(a(records)["status"], "PASS")
+        # Twin and nested sets never validate as each other.
+        self.assertEqual(self.mod.assess_nested(self.twin_set(), "r")["status"], "INVALID_SET")
+
+    def test_twin_runner_options(self):
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            base = ["run_case.py", "--repo", str(repo), "--expected-revision", "f" * 40,
+                    "--output", str(Path(tmp) / "case.json"), "--dry-run", "--re", "100"]
+            tw = ["--flow", "shear_wave", "--diagnostic", run_case.TWIN_SHEAR]
+            proj = ["--explosion", "projected_linear"]
+            for extra, valid in (
+                    (["--n", "128", "--depth", "1"] + tw, True),
+                    (["--n", "256", "--depth", "1"] + tw, True),
+                    (["--n", "128", "--depth", "2"] + tw, True),
+                    (["--n", "128", "--depth", "2"] + proj + tw, True),
+                    (["--n", "512", "--depth", "1"] + tw, False),
+                    (["--n", "128", "--depth", "3"] + tw, False),
+                    (["--n", "128", "--depth", "2"] + proj + tw + ["--final-snapshot"], False),
+                    (["--n", "128", "--depth", "2", "--diagnostic", run_case.TWIN_SHEAR], False)):
+                argv = base + extra
+                with self.subTest(argv=argv), patch.object(sys, "argv", argv), \
+                     patch.object(run_case, "verify_source", return_value={}) as verify, \
+                     patch.object(run_case, "runtime_source", return_value={}), \
+                     redirect_stdout(io.StringIO()), patch("sys.stderr", io.StringIO()):
+                    if valid:
+                        self.assertEqual(run_case.main(), 0)
+                        self.assertEqual(verify.call_args.args[2], "shear_twin")
+                    else:
+                        with self.assertRaises(SystemExit):
+                            run_case.main()
+
+    def test_mocked_twin_execution(self):
+        from types import SimpleNamespace
+        from contextlib import nullcontext
+        from CelerisLab.lbm.hierarchy import build_hierarchy
+        h = build_hierarchy(run_case.config(128, 100), self.twin.REFINEMENTS)
+        arrays = [sw.initial(k.shape, level, 128, U * 128 / sw.RE) for level, k in enumerate(h.kinds)]
+        selectors = []
+
+        class FakeRun:
+            def __init__(self, *args, **kwargs):
+                self.step_count = 0
+                if "refinements" in kwargs:
+                    selectors.append((kwargs["explosion"], kwargs["refinements"]))
+
+            def __enter__(self):
+                return self
+
+            def __exit__(self, *args):
+                pass
+
+            def initialize(self, *given):
+                assert len(given) == 2
+
+            def step(self, count):
+                self.step_count += count
+
+            def download(self):
+                return arrays
+
+        modules = {"CelerisLab.cuda.context": SimpleNamespace(CudaContext=FakeRun),
+                   "CelerisLab.lbm.block_stepper": SimpleNamespace(PeriodicBlockStepper=FakeRun),
+                   "CelerisLab.lbm.refinement_stepper": SimpleNamespace(FixedPatchStepper=FakeRun)}
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            output = Path(tmp) / "projected_twin.json"
+            argv = ["run_case.py", "--repo", str(repo), "--re", "100", "--n", "128", "--depth", "2",
+                    "--explosion", "projected_linear", "--flow", "shear_wave",
+                    "--diagnostic", run_case.TWIN_SHEAR, "--output", str(output),
+                    "--expected-revision", "f" * 40, "--run-gpu"]
+            with patch.dict(sys.modules, modules), patch.object(sys, "argv", argv), \
+                 patch.dict("os.environ", CUDA_VISIBLE_DEVICES="mock"), \
+                 patch.object(run_case, "verify_source", return_value=dict(revision="f" * 40)), \
+                 patch.object(run_case, "runtime_source", return_value={}), \
+                 patch.object(run_case, "runtime_identity", return_value={}), \
+                 patch.object(run_case, "build_capture", return_value=nullcontext()), \
+                 redirect_stdout(io.StringIO()):
+                run_case.main()
+            rec = json.loads(output.read_text())
+            self.assertTrue(rec["completed"])
+            self.assertEqual(rec["diagnostic"], run_case.TWIN_SHEAR)
+            self.assertEqual(rec["workload"]["active_cells_by_level"], self.twin.LEAVES)
+            self.assertEqual(rec["workload"]["active_collision_updates"], self.twin.WORK)
+            self.assertNotIn("final_snapshot", rec)
+            self.assertTrue(all(len(r["spurious_pressure_scaled_by_level"]) == 2 for r in rec["rows"]))
+            self.assertEqual(selectors, [("projected_linear", [list(self.twin.REFINEMENTS[0])])])
+            self.assertEqual(sorted(p.name for p in Path(tmp).iterdir()), ["projected_twin.json"])
+
+
+class ProbeTraceTests(unittest.TestCase):
+    """Signed interface-probe mechanism diagnostic (--probe-trace); no pass gates."""
+
+    def setUp(self):
+        import shear_nested
+        import shear_probes
+        import shear_twin
+        self.nested, self.probes, self.twin = shear_nested, shear_probes, shear_twin
+
+    def exact_arrays(self, masks, n, t):
+        """Equilibrium of the exact wave at root-physical time t on a case grid of root N=n."""
+        out = []
+        for level, m in enumerate(masks):
+            x, y = centers(m.shape, level)
+            x, y = x * 128 / n, y * 128 / n
+            ux, uy, _ = sw.exact(x, y, t, 128, U * 128 / sw.RE)
+            out.append(sw.equilibrium(np.ones_like(ux), ux, uy))
+        return out
+
+    def test_frozen_history_identities(self):
+        for name, digest in (("shear_wave.py", "81d5c8ee60864372887cddf9239580c82fb6750d956038f1e3ee9de61aa4e877"),
+                             ("shear_nested.py", "fa6cfe87fb4df8c6d732c637c885276833c81973b950c7724d734f8fc350bcb0"),
+                             ("shear_twin.py", "1cbf37f18e89aea577581631c4565f51e68d22a3739b698816ef64acbf2b2bb4"),
+                             ("oracle.py", ORACLE_SHA256_EEC4677)):
+            self.assertEqual(hashlib.sha256((HERE / name).read_bytes()).hexdigest(), digest, name)
+        self.assertEqual(run_case.SHEAR_PROBES_SHA256,
+                         hashlib.sha256((HERE / "shear_probes.py").read_bytes()).hexdigest())
+
+    def test_probe_selection_per_case(self):
+        sp = self.probes
+        self.assertEqual(len(sp.points()), 24)
+        corners = [(x, y) for level in (self.nested.REFINEMENTS[0],
+                                        [tuple(v / 2 for v in r) for r in self.nested.REFINEMENTS[1]])
+                   for x0, y0, x1, y1 in level for x in (x0, x1) for y in (y0, y1)]
+        for _, _, (x, y) in sp.points():
+            self.assertGreaterEqual(min(math.hypot(x - cx, y - cy) for cx, cy in corners), 8.0)
+        pattern = [0, 0, 0, 1, 1, 1, 1, 1, 2, 2, 2, 2]
+        nested = sp.select(self.nested.leaf_masks())
+        self.assertEqual([p["level"] for p in nested], pattern * 2)
+        twin = sp.select(self.twin.leaf_masks())
+        self.assertEqual([p["level"] for p in twin], [min(v, 1) for v in pattern] * 2)
+        u128 = sp.select([np.ones((128, 128), bool)], 128)
+        u512 = sp.select([np.ones((512, 512), bool)], 512)
+        self.assertTrue(all(p["level"] == 0 for p in u128 + u512))
+        for case in (nested, twin, u128, u512):
+            self.assertEqual(len({(p["level"], p["i"], p["j"]) for p in case}), 24)  # unique
+            for p in case:
+                dx, dy = p["displacement"]
+                self.assertEqual([p["center"][0] - p["point"][0], p["center"][1] - p["point"][1]],
+                                 [dx, dy])
+        self.assertEqual(max(math.hypot(*p["displacement"]) for p in u128), 0.625)
+        self.assertLessEqual(max(math.hypot(*p["displacement"]) for p in u512), 0.25 * 0.7072)
+        # Nested: 3 L0 + 5 L1 + 4 L2 per transect, as checked by native review.
+        for name in sp.TRANSECTS:
+            levels = [p["level"] for p in nested if p["transect"] == name]
+            self.assertEqual([levels.count(v) for v in (0, 1, 2)], [3, 5, 4])
+        # Different grids give different centers: no pretended collocation.
+        self.assertNotEqual(u128[3]["center"], u512[3]["center"])
+        self.assertEqual(sp.case_steps(512)[:3], [0, 4, 8])
+        self.assertEqual(sp.case_steps(128), list(sp.STEPS))
+
+    def test_residuals_vanish_for_the_exact_field(self):
+        sp = self.probes
+        for masks, n in ((self.nested.leaf_masks(), 128), ([np.ones((512, 512), bool)], 512)):
+            probes = sp.select(masks, n)
+            trace = sp.new_trace(probes, U * n / sw.RE, ["x", n, len(masks), "c"])
+            for t in (0, 100, 2048):
+                trace["steps"].append(t)
+                trace["values"].append(sp.sample(self.exact_arrays(masks, n, t), probes))
+            for res in sp.residuals(trace):
+                self.assertLess(max(abs(v) for r in res for v in r), 1e-14)
+
+    def test_validate_schedule_values_and_cap(self):
+        sp = self.probes
+        probes = sp.select(self.nested.leaf_masks())
+        trace = sp.new_trace(probes, 0.0384, ["n", 128, 3, "projected_linear"])
+        trace["steps"] = list(sp.STEPS)
+        trace["values"] = [[1.0, 0.01, -0.02] * 24 for _ in sp.STEPS]
+        size = sp.validate(trace)
+        self.assertLess(size, sp.TRACE_MAX_BYTES)
+        realistic = dict(trace, values=[[1.0000012345678901, 0.012345678901234567,
+                                         -0.023456789012345678] * 24 for _ in sp.STEPS])
+        self.assertLess(sp.validate(realistic), sp.TRACE_MAX_BYTES)
+        for bad in (dict(trace, steps=trace["steps"][:-1]),
+                    dict(trace, values=trace["values"][:-1] + [[float("nan")] * 72]),
+                    dict(trace, probes=trace["probes"][:-1])):
+            with self.assertRaises(ValueError):
+                sp.validate(bad)
+        with patch.object(sp, "TRACE_MAX_BYTES", 1000):
+            with self.assertRaises(ValueError):
+                sp.validate(trace)
+
+    def test_probe_runner_options(self):
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            base = ["run_case.py", "--repo", str(repo), "--expected-revision", "f" * 40,
+                    "--output", str(Path(tmp) / "case.json"), "--dry-run", "--re", "100",
+                    "--flow", "shear_wave", "--probe-trace"]
+            nest = ["--diagnostic", run_case.NESTED_SHEAR]
+            tw = ["--diagnostic", run_case.TWIN_SHEAR]
+            proj = ["--explosion", "projected_linear"]
+            for extra, valid in (
+                    (["--n", "128", "--depth", "1"] + nest, True),
+                    (["--n", "512", "--depth", "1"] + nest, True),
+                    (["--n", "128", "--depth", "3"] + proj + nest, True),
+                    (["--n", "128", "--depth", "2"] + proj + tw, True),
+                    (["--n", "128", "--depth", "3"] + nest, False),          # constant d3
+                    (["--n", "128", "--depth", "1"] + tw, False),
+                    (["--n", "128", "--depth", "3"] + proj + nest + ["--final-snapshot"], False),
+                    (["--n", "128", "--depth", "2"], False)):
+                argv = base + extra
+                with self.subTest(argv=argv), patch.object(sys, "argv", argv), \
+                     patch.object(run_case, "verify_source", return_value={}) as verify, \
+                     patch.object(run_case, "runtime_source", return_value={}), \
+                     redirect_stdout(io.StringIO()), patch("sys.stderr", io.StringIO()):
+                    if valid:
+                        self.assertEqual(run_case.main(), 0)
+                        self.assertIs(verify.call_args.kwargs["probes"], True)
+                    else:
+                        with self.assertRaises(SystemExit):
+                            run_case.main()
+
+    def run_mocked(self, n, depth, diagnostic, scheme, masks):
+        from types import SimpleNamespace
+        from contextlib import nullcontext
+        arrays = self.exact_arrays(masks, n, 0.0)
+        counts = []
+
+        class FakeRun:
+            def __init__(self, *args, **kwargs):
+                self.step_count = 0
+
+            def __enter__(self):
+                return self
+
+            def __exit__(self, *args):
+                pass
+
+            def initialize(self, *given):
+                pass
+
+            def step(self, count):
+                self.step_count += count
+                counts.append(self.step_count)
+
+            def download(self):
+                return arrays[0] if depth == 1 else arrays
+
+        modules = {"CelerisLab.cuda.context": SimpleNamespace(CudaContext=FakeRun),
+                   "CelerisLab.lbm.block_stepper": SimpleNamespace(PeriodicBlockStepper=FakeRun),
+                   "CelerisLab.lbm.refinement_stepper": SimpleNamespace(FixedPatchStepper=FakeRun)}
+        repo = Path(__file__).resolve().parents[3]
+        with tempfile.TemporaryDirectory() as tmp:
+            output = Path(tmp) / "probe.json"
+            argv = ["run_case.py", "--repo", str(repo), "--re", "100", "--n", str(n),
+                    "--depth", str(depth), "--explosion", scheme, "--flow", "shear_wave",
+                    "--diagnostic", diagnostic, "--probe-trace", "--output", str(output),
+                    "--expected-revision", "f" * 40, "--run-gpu"]
+            with patch.dict(sys.modules, modules), patch.object(sys, "argv", argv), \
+                 patch.dict("os.environ", CUDA_VISIBLE_DEVICES="mock"), \
+                 patch.object(run_case, "verify_source", return_value=dict(revision="f" * 40)), \
+                 patch.object(run_case, "runtime_source", return_value={}), \
+                 patch.object(run_case, "runtime_identity", return_value={}), \
+                 patch.object(run_case, "build_capture", return_value=nullcontext()), \
+                 redirect_stdout(io.StringIO()):
+                run_case.main()
+            return json.loads(output.read_text()), counts, sorted(p.name for p in Path(tmp).iterdir())
+
+    def test_mocked_probe_traces(self):
+        sp = self.probes
+        rec, counts, files = self.run_mocked(128, 3, run_case.NESTED_SHEAR, "projected_linear",
+                                             self.nested.leaf_masks())
+        trace = rec["probe_trace"]
+        self.assertEqual(trace["steps"], list(sp.STEPS))
+        self.assertEqual(len(trace["values"]), len(sp.STEPS))
+        self.assertLessEqual(trace["bytes"], sp.TRACE_MAX_BYTES)
+        self.assertEqual(trace["case"], [run_case.NESTED_SHEAR, 128, 3, "projected_linear"])
+        self.assertEqual(len(rec["rows"]), 5)  # qualification samples preserved
+        self.assertEqual(files, ["probe.json"])  # no field dumps
+        self.assertEqual(sp.validate(trace) <= sp.TRACE_MAX_BYTES, True)
+        rec512, counts512, _ = self.run_mocked(512, 1, run_case.NESTED_SHEAR, "constant",
+                                               [np.ones((512, 512), bool)])
+        t512 = rec512["probe_trace"]
+        self.assertEqual(t512["steps"], list(sp.STEPS))  # root-physical times
+        self.assertEqual(t512["viscosity_root"], U * 128 / sw.RE)
+        self.assertEqual(t512["case_viscosity"], U * 512 / sw.RE)
+        self.assertIn(4 * 1920, counts512)  # U512 advanced to 4x lattice steps
+        self.assertTrue(all(p["level"] == 0 for p in t512["probes"]))
+        res = sp.residuals(t512)
+        self.assertLess(abs(res[0][0][1]), 1e-14)  # exact field at t=0 at actual centers