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/CMakeLists.txt

@@ -3,6 +3,8 @@ add_library(
     STATIC
     analysis/analysis_model.cpp
     analysis/analysis_state.cpp
+    analysis/eigen_reduction.cpp
+    analysis/linear_frequency_procedure.cpp
     analysis/linear_static_analysis.cpp
     analysis/linear_static_procedure.cpp
     analysis/newton_iteration_log.cpp
@@ -42,6 +44,8 @@ add_library(
     io/hdf5/hdf5_primitives.cpp
     io/hdf5/hdf5_result_writer.cpp
     io/hdf5/hdf5_results_writer.cpp
+    io/hdf5/eigen_hdf5_results_writer.cpp
+    io/hdf5/eigen_hdf5_self_check.cpp
     io/hdf5/hdf5_self_check.cpp
     loads/concentrated_nodal_load.cpp
     loads/shell_surface_pressure.cpp
@@ -61,6 +65,7 @@ add_library(
     results/recovery_candidate.cpp
     results/result_recovery.cpp
     results/shell_result_recovery.cpp
+    solvers/eigen/mkl_feast_eigen_solver.cpp
     solvers/linear/mkl_pardiso_solver.cpp
     solvers/linear/mkl_pardiso_tangent_solver.cpp
 )