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/model/domain.h

@@ -4,6 +4,7 @@
 #include <cstddef>
 #include <filesystem>
 #include <memory>
+#include <optional>
 #include <string>
 #include <vector>
 
@@ -47,7 +48,7 @@ class DomainCollectionView {
   std::vector<const T*> entries_;
 };
 
-/// @brief Owns one immutable static-step semantic definition.
+/// @brief Owns one immutable static or frequency-step semantic definition.
 /// @note Loads retain source order and are owned polymorphically by unique
 /// pointers.
 class StepDefinition {
@@ -84,6 +85,12 @@ class StepDefinition {
   /// @brief Reports whether this step uses the nonlinear static procedure.
   bool IsNonlinearStatic() const noexcept;
 
+  /// @brief Reports whether this step defines the frequency procedure.
+  bool IsFrequency() const noexcept;
+
+  /// @brief Returns frequency source semantics when this is a frequency step.
+  const FrequencyStepDefinition* Frequency() const noexcept;
+
   /// @brief Reports the source NLGEOM setting for this static step.
   bool UsesNonlinearGeometry() const noexcept;
 
@@ -108,6 +115,9 @@ class StepDefinition {
   /// @brief Converts one parsed static-step record to owned semantic objects.
   explicit StepDefinition(StaticStepDefinition definition);
 
+  /// @brief Converts one parsed frequency-step record to owned semantics.
+  explicit StepDefinition(FrequencyStepDefinition definition);
+
   std::string name_;
   std::vector<std::unique_ptr<BoundaryCondition>> boundary_conditions_;
   BoundaryConditionView boundary_conditions_view_;
@@ -124,6 +134,7 @@ class StepDefinition {
   double minimum_increment_;
   double maximum_increment_;
   SourceLocation location_;
+  std::optional<FrequencyStepDefinition> frequency_;
 };
 
 /// @brief Owns the complete immutable semantic model definition.
@@ -184,9 +195,15 @@ class Domain {
   /// @brief Returns assembly element surfaces in stable declaration order.
   const std::vector<ElementSurfaceDefinition>& ElementSurfaces() const noexcept;
 
-  /// @brief Returns static steps in stable declaration order.
+  /// @brief Returns procedure steps in stable declaration order.
   const DomainCollectionView<StepDefinition>& Steps() const noexcept;
 
+  /// @brief Returns resolved density records in stable material-index order.
+  const std::vector<MaterialDensityDefinition>& Densities() const noexcept;
+
+  /// @brief Returns the sole frequency source definition when present.
+  const FrequencyStepDefinition* Frequency() const noexcept;
+
   /// @brief Returns sorted nonfatal mapping diagnostics.
   const std::vector<Diagnostic>& Warnings() const noexcept;