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.

src/fesa/analysis/hht_reduced_procedure.h

@@ -0,0 +1,27 @@
+#ifndef FESA_ANALYSIS_HHT_REDUCED_PROCEDURE_H_
+#define FESA_ANALYSIS_HHT_REDUCED_PROCEDURE_H_
+
+#include <cstdint>
+
+#include "fesa/analysis/prepared_analysis.h"
+
+namespace fesa {
+namespace hht_reduced_internal {
+constexpr std::uint32_t kMaximumOuterCorrections = 100U;
+constexpr bool CanApplyOuterCorrection(const std::uint32_t completed) {
+  return completed < kMaximumOuterCorrections;
+}
+}  // namespace hht_reduced_internal
+
+class LinearSolver;
+class NewtonIterationLogWriter;
+class ParallelFor;
+class TangentSolver;
+
+Result<DynamicExecutionResult> ExecuteReducedHht(
+    const Domain& domain, NewtonIterationLogWriter& log,
+    const ParallelFor& parallel_for, LinearSolver& positive_solver,
+    TangentSolver& tangent_solver);
+}  // namespace fesa
+
+#endif  // FESA_ANALYSIS_HHT_REDUCED_PROCEDURE_H_