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.

tests/reference/eigen_reference_comparison.h

@@ -0,0 +1,49 @@
+#ifndef FESA_TESTS_REFERENCE_EIGEN_REFERENCE_COMPARISON_H_
+#define FESA_TESTS_REFERENCE_EIGEN_REFERENCE_COMPARISON_H_
+
+#include <cstdint>
+#include <filesystem>
+#include <string>
+#include <vector>
+
+#include "fesa/core/status.h"
+#include "reference_tolerance_policy.h"
+
+namespace fesa::test {
+
+struct EigenReferenceCase {
+  std::string case_id;
+  std::filesystem::path input_path;
+  std::filesystem::path eigenvalues_text_path;
+  std::filesystem::path results_hdf5_path;
+};
+
+struct EigenReferenceRowDecision {
+  std::string source_step_name;
+  std::uint64_t mode_id;
+  std::string component;
+  double fesa_value;
+  double reference_value;
+  ReferenceToleranceRowDecision tolerance;
+};
+
+struct EigenReferenceComparisonReport {
+  std::string case_id;
+  std::string source_step_name;
+  std::uint64_t requested_mode_count;
+  std::vector<EigenReferenceRowDecision> rows;
+  std::vector<ReferenceToleranceFamilyDecision> families;
+  bool passed;
+};
+
+/// @brief Compares modal eigenvalues without changing the approved input or
+/// TXT.
+class EigenReferenceComparison {
+ public:
+  static Result<EigenReferenceComparisonReport> Compare(
+      const EigenReferenceCase& reference_case);
+};
+
+}  // namespace fesa::test
+
+#endif  // FESA_TESTS_REFERENCE_EIGEN_REFERENCE_COMPARISON_H_