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/results/eigen_results_writer.h

@@ -0,0 +1,29 @@
+#ifndef FESA_RESULTS_EIGEN_RESULTS_WRITER_H_
+#define FESA_RESULTS_EIGEN_RESULTS_WRITER_H_
+
+#include <filesystem>
+#include <vector>
+
+#include "fesa/analysis/eigen_analysis_state.h"
+#include "fesa/core/diagnostic.h"
+#include "fesa/core/status.h"
+
+namespace fesa {
+
+class Domain;
+
+/// @brief Isolates authoritative modal storage from procedure execution.
+class EigenResultsWriter {
+ public:
+  virtual ~EigenResultsWriter() = default;
+
+  /// @brief Validates and atomically publishes the complete modal candidate.
+  /// @return Success only after closed-file validation and final replacement.
+  virtual Status Write(const std::filesystem::path& output_path,
+                       const Domain& domain, const EigenAnalysisState& state,
+                       const std::vector<Diagnostic>& diagnostics) = 0;
+};
+
+}  // namespace fesa
+
+#endif  // FESA_RESULTS_EIGEN_RESULTS_WRITER_H_