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

include/fesa/assembly/load_assembler.h

@@ -13,17 +13,21 @@ namespace fesa {
 class LoadAssembler {
  public:
   /// @brief Accumulates all active CLOAD rows in full-DOF space.
+  /// @param step_time Actual finite nonnegative dynamic endpoint for amplitude.
   /// @return A finite full load vector or a structured model failure.
   static Result<Vector> AssembleFullNodalLoad(const AnalysisModel& model,
-                                              const DofManager& dofs);
+                                              const DofManager& dofs,
+                                              double step_time = 0.0);
 
   /// @brief Accumulates explicitly supplied loads in their view order.
   /// @param loads Non-owning loads whose contribution source orders must match
   /// their view positions.
+  /// @param step_time Actual finite nonnegative dynamic endpoint for amplitude.
   /// @return A candidate committed only after all contributions validate.
   static Result<Vector> AssembleFullNodalLoad(const AnalysisModel& model,
                                               const DofManager& dofs,
-                                              const LoadView& loads);
+                                              const LoadView& loads,
+                                              double step_time = 0.0);
 
   /// @brief Forms Ff-Kfc*dc in stable free/constrained order.
   /// @note This operation neither factorizes nor invokes a solver.