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/run_case.py

@@ -18,7 +18,7 @@ SOURCE_SHA256 = "a5dbfd804befce868067d79a23e5fd47dce7f505b3f9b1bb2145fb57cb5f217
 BUILD_CAPTURE_SHA256 = "3dbaa6547cb0a4cb1541054f3b64ad8b6e18cc70527ac6166e56b944e9bd464a"
 REFERENCE_HARNESS = "11901a7f77557a901e017a171eeae1758d584fa5"
 INITIALIZATION = "physical FP64 equilibrium G at actual cell centers; rho=1+3p"
-SHEAR_WAVE_SHA256 = "4597cf1870c1a69b4525c231825a58cf601b3fe22efd2aecf8fca76601c972e8"
+SHEAR_WAVE_SHA256 = "81d5c8ee60864372887cddf9239580c82fb6750d956038f1e3ee9de61aa4e877"
 
 
 def flow_functions(flow="taylor_green"):