test: opt-in scaled_shear_n256 four-case shear diagnostic

- run_case: --diagnostic scaled_shear_n256 (requires --flow shear_wave;
  U256/U512/constant R256/projected R256, Re100, no early trace); N512
  only via one of the two scaled diagnostics; scaled_n256 stays TG-only.
- assess_shear_wave: scaled=True contract reusing 92b8afe scaled
  case/build/workload checks, shear diagnostic identity; signed rate bias,
  refined excess over each control; hash-pinned de5b0cc N128 reference and
  descriptive N128->N256 signed trend; CLI --diagnostic/--reference-files.
- tests: scaled contract, set mixing, trend/reference pin, runner options.
- shear_wave.py and oracle.py unchanged.

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

tests/validation/taylor_green/run_case.py

@@ -20,6 +20,7 @@ REFERENCE_HARNESS = "11901a7f77557a901e017a171eeae1758d584fa5"
 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)
 
 
 def flow_functions(flow="taylor_green"):
@@ -181,8 +182,9 @@ 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],
-                   help="explicit four-case N256/fine512 same-placement diagnostic")
+    p.add_argument("--diagnostic", choices=[SCALED_DIAGNOSTIC, SCALED_SHEAR],
+                   help="explicit four-case N256/fine512 same-placement diagnostic "
+                        "(scaled_n256: Taylor-Green; scaled_shear_n256: shear wave)")
     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")
@@ -197,12 +199,17 @@ def main():
     if args.diagnostic:
         cases = {(256, 1, "constant"), (512, 1, "constant"),
                  (256, 2, "constant"), (256, 2, "projected_linear")}
-        if args.flow != "taylor_green":
-            p.error("--diagnostic scaled_n256 requires --flow taylor_green")
-        if args.re != 100 or (args.n, args.depth, args.explosion) not in cases or not args.early_pressure:
+        flow = "shear_wave" if args.diagnostic == SCALED_SHEAR else "taylor_green"
+        if args.flow != flow:
+            p.error(f"--diagnostic {args.diagnostic} requires --flow {flow}")
+        if args.diagnostic == SCALED_DIAGNOSTIC and (
+                args.re != 100 or (args.n, args.depth, args.explosion) not in cases
+                or not args.early_pressure):
             p.error("scaled_n256 requires one of its four Re100 cases and --early-pressure")
+        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")
+        p.error("N512 requires --diagnostic scaled_n256 or scaled_shear_n256")
     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")