feat: map fixed-step HHT shell dynamics input

include/fesa/model/domain.h

@@ -88,9 +88,15 @@ class StepDefinition {
   /// @brief Reports whether this step defines the frequency procedure.
   bool IsFrequency() const noexcept;
 
+  /// @brief Reports whether this step defines the HHT dynamic procedure.
+  bool IsDynamic() const noexcept;
+
   /// @brief Returns frequency source semantics when this is a frequency step.
   const FrequencyStepDefinition* Frequency() const noexcept;
 
+  /// @brief Returns dynamic source semantics when this is a dynamic step.
+  const DynamicStepDefinition* Dynamic() const noexcept;
+
   /// @brief Reports the source NLGEOM setting for this static step.
   bool UsesNonlinearGeometry() const noexcept;
 
@@ -118,6 +124,9 @@ class StepDefinition {
   /// @brief Converts one parsed frequency-step record to owned semantics.
   explicit StepDefinition(FrequencyStepDefinition definition);
 
+  /// @brief Converts one parsed dynamic-step record to owned semantics.
+  explicit StepDefinition(DynamicStepDefinition definition);
+
   std::string name_;
   std::vector<std::unique_ptr<BoundaryCondition>> boundary_conditions_;
   BoundaryConditionView boundary_conditions_view_;
@@ -135,6 +144,7 @@ class StepDefinition {
   double maximum_increment_;
   SourceLocation location_;
   std::optional<FrequencyStepDefinition> frequency_;
+  std::optional<DynamicStepDefinition> dynamic_;
 };
 
 /// @brief Owns the complete immutable semantic model definition.
@@ -204,6 +214,9 @@ class Domain {
   /// @brief Returns the sole frequency source definition when present.
   const FrequencyStepDefinition* Frequency() const noexcept;
 
+  /// @brief Returns the sole dynamic source definition when present.
+  const DynamicStepDefinition* Dynamic() const noexcept;
+
   /// @brief Returns sorted nonfatal mapping diagnostics.
   const std::vector<Diagnostic>& Warnings() const noexcept;