feat(solid): add Hex8 input and three-DOF semantics

include/fesa/model/domain.h

@@ -112,6 +112,13 @@ class StepDefinition {
   /// @brief Returns the static maximum-increment provenance value.
   double MaximumIncrement() const noexcept;
 
+  /// @brief Reports the source STATIC DIRECT fixed-schedule selection.
+  bool IsDirectStatic() const noexcept;
+  /// @brief Returns the fixed Solid static increment count, or zero otherwise.
+  std::uint32_t IncrementCount() const noexcept;
+  /// @brief Returns the source static INC limit.
+  std::uint32_t MaximumIncrements() const noexcept;
+
   /// @brief Returns the source location of the step declaration.
   const SourceLocation& Location() const noexcept;
 
@@ -145,6 +152,9 @@ class StepDefinition {
   SourceLocation location_;
   std::optional<FrequencyStepDefinition> frequency_;
   std::optional<DynamicStepDefinition> dynamic_;
+  bool direct_static_{false};
+  std::uint32_t increment_count_{0U};
+  std::uint32_t maximum_increments_{100U};
 };
 
 /// @brief Owns the complete immutable semantic model definition.
@@ -176,6 +186,15 @@ class Domain {
   const DomainCollectionView<Mitc4ShellDefinition>& ShellElements()
       const noexcept;
 
+  /// @brief Returns C3D8 definitions in stable source order.
+  const DomainCollectionView<Hex8Definition>& SolidElements() const noexcept;
+  /// @brief Returns homogeneous Solid section assignments.
+  const DomainCollectionView<SolidSection>& SolidSections() const noexcept;
+  /// @brief Reports whether this model has exclusively Solid elements.
+  bool IsSolid() const noexcept;
+  /// @brief Returns immutable model-level amplitude curves in source order.
+  const std::vector<TabularAmplitudeDefinition>& Amplitudes() const noexcept;
+
   /// @brief Returns all materials in stable Domain index order.
   const DomainCollectionView<Material>& Materials() const noexcept;
 
@@ -262,6 +281,8 @@ class Domain {
   DomainCollectionView<ElementDefinition> elements_view_;
   DomainCollectionView<EulerBeam3DDefinition> beam_elements_view_;
   DomainCollectionView<Mitc4ShellDefinition> shell_elements_view_;
+  DomainCollectionView<Hex8Definition> solid_elements_view_;
+  DomainCollectionView<SolidSection> solid_sections_view_;
   DomainCollectionView<ElementProperty> properties_view_;
   DomainCollectionView<GeneralBeamSection> sections_view_;
   DomainCollectionView<ShellSection> shell_sections_view_;