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.

tests/unit/io/abaqus/domain_mapping_components_test.cpp

@@ -157,6 +157,9 @@ TEST(DomainMappingComponents,
   EXPECT_EQ(domain.Steps()[0].Loads().size(), 1U);
   ASSERT_EQ(domain.Warnings().size(), 3U);
   EXPECT_EQ(domain.Warnings()[0].keyword, "PREPRINT");
+  EXPECT_EQ(
+      domain.Warnings()[0].message,
+      "The allowlisted Abaqus keyword is ignored without semantic effect.");
   EXPECT_EQ(domain.Warnings()[1].keyword, "OUTPUT");
   EXPECT_EQ(domain.Warnings()[2].keyword, "NODE OUTPUT");
 }