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.

src/fesa/io/hdf5/eigen_hdf5_self_check.h

@@ -0,0 +1,24 @@
+#ifndef FESA_IO_HDF5_EIGEN_HDF5_SELF_CHECK_H_
+#define FESA_IO_HDF5_EIGEN_HDF5_SELF_CHECK_H_
+
+#include <filesystem>
+#include <vector>
+
+#include "fesa/analysis/eigen_analysis_state.h"
+#include "fesa/core/status.h"
+#include "fesa/model/domain.h"
+
+namespace fesa {
+struct Diagnostic;
+}
+
+namespace fesa::hdf5_internal {
+
+/// @brief Reopens and validates a closed authoritative eigen HDF5 candidate.
+Status SelfCheckEigenFile(const std::filesystem::path& path,
+                          const Domain& domain, const EigenAnalysisState& state,
+                          const std::vector<Diagnostic>& diagnostics);
+
+}  // namespace fesa::hdf5_internal
+
+#endif  // FESA_IO_HDF5_EIGEN_HDF5_SELF_CHECK_H_