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>

docs/validation/taylor_green.md

@@ -262,6 +262,11 @@ Run set: uniform N, uniform 2N, and refined depth-2 runs at root N.
     `controlled_pass` is false, so it is never a controlled-comparison pass;
   - PASS/FAIL: every refined run's `controlled_pass` (own screens and
     budgets against valid, passing controls).
+  - Finite modal extinction (late R = 0) is a physical failure, not
+    malformed data. The run gets `rate_fit_valid=false` with null fit
+    quantities, and its other metrics are kept. Nothing is clipped and no
+    rate is invented. The result is CONTROL_SCREEN_FAIL for a control, or
+    FAIL for a refined run.
   - Screens are never retuned.
 
 ### Limitations