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/model/domain.cpp

@@ -44,6 +44,31 @@ StepDefinition::StepDefinition(StaticStepDefinition definition)
   surface_pressure_loads_ = std::move(definition.surface_pressure_loads);
 }
 
+StepDefinition::StepDefinition(FrequencyStepDefinition definition)
+    : name_{definition.source_step_name_raw},
+      procedure_kind_{StaticProcedureKind::kLinearSmallDisplacement},
+      nlgeom_{false},
+      initial_increment_{0.0},
+      time_period_{0.0},
+      minimum_increment_{0.0},
+      maximum_increment_{0.0},
+      location_{definition.step_location},
+      frequency_{std::move(definition)} {
+  for (const auto& boundary : frequency_->boundaries) {
+    for (int source_dof = boundary.first_dof; source_dof <= boundary.last_dof;
+         ++source_dof) {
+      const std::size_t source_order = boundary_conditions_.size();
+      auto owned = std::make_unique<PrescribedDisplacementBoundaryCondition>(
+          SourceTargetQuery{SourceEntityKind::kNode, "", boundary.target},
+          static_cast<DofComponent>(source_dof - 1), boundary.value,
+          source_order, boundary.location);
+      boundary_conditions_view_.push_back(std::cref(*owned));
+      prescribed_displacements_view_.Add(*owned);
+      boundary_conditions_.push_back(std::move(owned));
+    }
+  }
+}
+
 const std::string& StepDefinition::Name() const noexcept { return name_; }
 
 const BoundaryConditionView& StepDefinition::BoundaryConditions()
@@ -76,6 +101,14 @@ bool StepDefinition::IsNonlinearStatic() const noexcept {
   return procedure_kind_ == StaticProcedureKind::kNonlinearFullNewton;
 }
 
+bool StepDefinition::IsFrequency() const noexcept {
+  return frequency_.has_value();
+}
+
+const FrequencyStepDefinition* StepDefinition::Frequency() const noexcept {
+  return frequency_ ? &*frequency_ : nullptr;
+}
+
 bool StepDefinition::UsesNonlinearGeometry() const noexcept { return nlgeom_; }
 
 double StepDefinition::InitialIncrement() const noexcept {
@@ -165,6 +198,15 @@ const DomainCollectionView<StepDefinition>& Domain::Steps() const noexcept {
   return steps_view_;
 }
 
+const std::vector<MaterialDensityDefinition>& Domain::Densities()
+    const noexcept {
+  return definition_.densities;
+}
+
+const FrequencyStepDefinition* Domain::Frequency() const noexcept {
+  return steps_view_.Size() == 1U ? steps_view_[0U].Frequency() : nullptr;
+}
+
 const std::vector<Diagnostic>& Domain::Warnings() const noexcept {
   return definition_.warnings;
 }
@@ -227,7 +269,8 @@ Domain::Domain(ModelDefinition definition)
   }
   definition_.shell_elements.clear();
 
-  step_definitions_.reserve(definition_.steps.size());
+  step_definitions_.reserve(definition_.steps.size() +
+                            (definition_.frequency_step ? 1U : 0U));
   for (auto& step : definition_.steps) {
     auto owned =
         std::unique_ptr<StepDefinition>(new StepDefinition(std::move(step)));
@@ -235,6 +278,12 @@ Domain::Domain(ModelDefinition definition)
     step_definitions_.push_back(std::move(owned));
   }
   definition_.steps.clear();
+  if (definition_.frequency_step) {
+    auto owned = std::unique_ptr<StepDefinition>(
+        new StepDefinition(std::move(*definition_.frequency_step)));
+    steps_view_.Add(*owned);
+    step_definitions_.push_back(std::move(owned));
+  }
 }
 
 }  // namespace fesa