test: shear-wave modal-extinction handling; TG mock verify_source flow arg

- shear_wave.fit_rate: finite late R=0 returns rate_fit_valid=false with
  null fit quantities (no clipping, no invented rate); own screens and
  budgets treat it as a failure (CONTROL_SCREEN_FAIL / refined FAIL),
  other finite metrics retained, JSON-safe.
- test_projected_pairs: dry-run/execution mocks expect the explicit
  'taylor_green' flow argument now passed to verify_source.
- run_case: SHEAR_WAVE_SHA256 repinned; extinction test added.

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

tests/validation/taylor_green/shear_wave.py

@@ -155,15 +155,21 @@ def fit_rate(rows, length, viscosity):
     """
     late = [r for r in rows if r["tstar"] >= FIT_FROM_TSTAR - 1e-12]
     amp = np.array([math.hypot(r["a_hat"], r["b_hat"]) for r in late])
-    if len(late) < 3 or not (np.isfinite(amp).all() and (amp > 0).all()):
-        raise ValueError("need >= 3 late samples with finite positive R")
+    if len(late) < 3 or not np.isfinite(amp).all():
+        raise ValueError("need >= 3 late samples with finite R")
+    exact_rate = decay_rate(length, viscosity)
+    to_tstar = length / U  # root steps per unit t*
+    if not (amp > 0).all():
+        # Finite modal extinction: a physical failure, not malformed data. No
+        # clipping and no invented rate; fit quantities are unavailable.
+        return dict(rate_fit_valid=False, rate=None, exact_rate=exact_rate,
+                    rate_tstar=None, exact_rate_tstar=exact_rate * to_tstar,
+                    rate_error=None, interval_rate_errors=None, log_fit_residual_max=None)
     t = np.array([r["step"] for r in late], dtype=float)
     (slope, intercept) = np.polyfit(t, np.log(amp), 1)
     residual = np.log(amp) - (slope * t + intercept)
-    exact_rate = decay_rate(length, viscosity)
-    to_tstar = length / U  # root steps per unit t*
     return dict(
-        rate=float(-slope), exact_rate=exact_rate,
+        rate_fit_valid=True, rate=float(-slope), exact_rate=exact_rate,
         rate_tstar=float(-slope * to_tstar),
         exact_rate_tstar=exact_rate * to_tstar,  # = 5 (2 pi)^2 / Re
         rate_error=abs(-slope / exact_rate - 1),
@@ -228,10 +234,11 @@ def summarize(record):
         velocity_l2=max(r["velocity_l2"] for r in rows),
         mass_relative=max(r["mass_relative"] for r in rows),
     )
-    if not all(_number(v) for v in (out["rate"], out["rate_error"], *out["interval_rate_errors"],
-                                     out["log_fit_residual_max"])):
+    if out["rate_fit_valid"] and not all(
+            _number(v) for v in (out["rate"], out["rate_error"], *out["interval_rate_errors"],
+                                 out["log_fit_residual_max"])):
         raise ValueError("nonfinite rate fit")
-    out["own_screens_pass"] = out["complete"] and all(
+    out["own_screens_pass"] = out["complete"] and out["rate_fit_valid"] and all(
         out[key] <= out["eps"] for key in ("rate_error", "phase_drift_abs", "residual_l2")
     ) and all(out[key] <= limit for key, limit in LIMITS.items())
     return out
@@ -280,12 +287,16 @@ def assess(base_control, fine_control, refined):
             statuses.append("INVALID_RUN")
             continue
         s["valid"] = True
+        # rate_error is None when a modal fit is unavailable; never a pass.
         s["budget"] = {
-            key: max(BASELINE_FACTOR * base[key], floor) for key, floor in FLOORS.items()
+            key: (None if base[key] is None else max(BASELINE_FACTOR * base[key], floor))
+            for key, floor in FLOORS.items()
         }
-        s["budget_pass"] = all(s[key] <= s["budget"][key] for key in FLOORS)
+        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 FLOORS)
         s["control_ratios"] = {
-            name: {key: (s[key] / c[key] if c[key] > 0 else None)
+            name: {key: (s[key] / c[key] if s[key] is not None and c[key] else None)
                    for key in (*FLOORS, "spurious_pressure_rms")}
             for name, c in (("root", base), ("finest", fine))
         }