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/prepared_analysis.h

@@ -6,6 +6,7 @@
 #include <vector>
 
 #include "fesa/analysis/analysis_state.h"
+#include "fesa/analysis/eigen_analysis_state.h"
 #include "fesa/analysis/nonlinear_analysis_state.h"
 #include "fesa/core/diagnostic.h"
 #include "fesa/core/status.h"
@@ -14,6 +15,8 @@
 namespace fesa {
 
 class LinearStaticProcedureExecutor;
+class LinearFrequencyProcedureExecutor;
+class EigenResultsWriter;
 class NewtonIterationLogWriter;
 class NonlinearResultsWriter;
 class NonlinearStaticProcedureExecutor;
@@ -47,7 +50,8 @@ struct NonlinearStaticExecutionResult {
 };
 
 using PreparedAnalysisOutcome =
-    std::variant<LinearStaticExecutionResult, NonlinearStaticExecutionResult>;
+    std::variant<LinearStaticExecutionResult, NonlinearStaticExecutionResult,
+                 EigenAnalysisState>;
 
 /// @brief Selects the approved procedure from a prepared Domain.
 /// @pre The executors and log outlive this call.
@@ -55,6 +59,7 @@ using PreparedAnalysisOutcome =
 Result<PreparedAnalysisOutcome> ExecutePreparedAnalysis(
     const Domain& domain, LinearStaticProcedureExecutor& linear_executor,
     NonlinearStaticProcedureExecutor& nonlinear_executor,
+    const LinearFrequencyProcedureExecutor& frequency_executor,
     NewtonIterationLogWriter& log);
 
 /// @brief Writes an already computed outcome to its authoritative result file.
@@ -64,7 +69,7 @@ Status FinalizePreparedAnalysisResult(
     const DerivedOutputPaths& paths, const Domain& domain,
     const PreparedAnalysisOutcome& outcome,
     const std::vector<Diagnostic>& diagnostics, ResultsWriter& linear_writer,
-    NonlinearResultsWriter& nonlinear_writer);
+    NonlinearResultsWriter& nonlinear_writer, EigenResultsWriter& eigen_writer);
 
 }  // namespace fesa