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/elements/mitc4_shell.h

@@ -98,6 +98,14 @@ struct Mitc4Stiffness {
   double drilling_stiffness;
 };
 
+/// @brief Stores consistent scalar moments and their derived MITC4 masses.
+struct Mitc4Mass {
+  std::array<Matrix, 3> consistent_moments;
+  Matrix consistent_global24;
+  Matrix lumped_global24;
+  std::array<std::array<double, 3>, 4> nodal_moments;
+};
+
 /// @brief Stores physical shell recovery at one midsurface Gauss location.
 struct Mitc4PhysicalRecoveryPoint {
   std::array<double, 2> natural_coordinates;
@@ -187,6 +195,11 @@ class Mitc4Shell final : public Element {
   /// @return Finite stiffness matrices or a structured model failure.
   [[nodiscard]] Result<Mitc4Stiffness> Stiffness() const;
 
+  /// @brief Computes consistent and scalar-row-sum lumped MITC4 mass.
+  /// @param density Constant material density in mass per volume.
+  /// @return Finite mass matrices or a structured model failure.
+  [[nodiscard]] Result<Mitc4Mass> Mass(double density) const;
+
   /// @brief Recovers physical shell quantities without drilling results.
   /// @param global_element_displacement24 Global element DOFs in node order.
   /// @return Physical recovery rows or a structured model failure.