test: fixed three-level/two-region nested shear preset (nested_shear_n128)

Test-only opt-in. New shear_nested.py: fixed parent-frame rectangles
(L0->L1 (16,16,60,112),(68,24,112,120); L1->L2 (44,44,108,212),
(148,60,212,228)), exact leaves/hops/work/nu/tau, depth-3 + 4N-finest
generalization of the shear assessment (same summaries/screens/budgets),
bounded final macro snapshot writer/reader (packed ACTIVE leaves + rho/u,
metadata, sha256, 8 MiB cap; no populations/restart/time series).
run_case: --diagnostic nested_shear_n128 (U128, U512, constant/projected
R128 depth 3), --final-snapshot only for projected depth 3, shear_nested.py
path+hash pin. assess_shear_wave: assess_nested contract and CLI.
shear_wave.py and oracle.py byte-identical; old guards/defaults unchanged.

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

docs/validation/taylor_green.md

@@ -342,3 +342,46 @@ Assessment:
   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.