feat: add eigen2 frequency pipeline and fail-closed validation

Implement MITC4 mass, constrained whitening, MKL FEAST, modal HDF5, and deterministic Value comparison. MSVC Debug builds; 394 of 395 tests pass. Original-model all-c residual and the documented S3 static contract conflict remain unresolved; this is an implementation checkpoint, not feature acceptance.

include/fesa/analysis/linear_frequency_procedure.h

@@ -0,0 +1,20 @@
+#ifndef FESA_ANALYSIS_LINEAR_FREQUENCY_PROCEDURE_H_
+#define FESA_ANALYSIS_LINEAR_FREQUENCY_PROCEDURE_H_
+
+#include "fesa/analysis/eigen_analysis_state.h"
+
+namespace fesa {
+
+class Domain;
+
+/// @brief Executes the supported linear MITC4 frequency procedure.
+class LinearFrequencyProcedureExecutor final {
+ public:
+  /// @brief Builds, solves, and validates one frequency modal candidate.
+  /// @return Requested physical modes plus all-c validation evidence.
+  Result<EigenAnalysisState> Execute(const Domain& domain) const;
+};
+
+}  // namespace fesa
+
+#endif  // FESA_ANALYSIS_LINEAR_FREQUENCY_PROCEDURE_H_