feat: adopt lumped mass and reduced-coordinate shell HHT

Integrate Hex8 mass verification and fixed-null-basis shell dynamics with numerical and physics handoff evidence. Preserve interrupted reference and full-validation status; full integration validation is deferred until after performance optimization.

tests/CMakeLists.txt

@@ -14,6 +14,7 @@ add_executable(
     unit/analysis/hht_energy_verification_test.cpp
     unit/analysis/hht_temporal_verification_test.cpp
     unit/analysis/hht_dynamic_algebra_test.cpp
+    unit/analysis/hht_reduced_shell_test.cpp
     unit/analysis/hht_dynamic_procedure_test.cpp
     unit/analysis/nonlinear_analysis_state_test.cpp
     unit/analysis/nonlinear_static_control_test.cpp