fix: enforce thermal boundary dof endpoints

include/fesa/model/model_types.h

@@ -239,6 +239,8 @@ struct ThermalMaterialDefinition {
 };
 
 /// @brief Stores one centered homogeneous thermal shell assignment.
+/// @note material_index addresses ModelDefinition::thermal_materials and,
+/// after Domain construction, Domain::ThermalMaterials(), never Properties().
 struct ThermalShellSectionDefinition {
   SourceEntityId source_id;
   std::string name;
@@ -268,6 +270,16 @@ struct ThermalInitialConditionDefinition {
   SourceLocation location;
 };
 
+/// @brief Identifies the fixed bottom-to-top thermal station source DOFs.
+/// @note This is separate from the six-component structural DofComponent.
+enum class ThermalDofComponent : int {
+  kBottom = 11,
+  kLower = 12,
+  kMiddle = 13,
+  kUpper = 14,
+  kTop = 15
+};
+
 /// @brief Stores one prescribed thermal source DOF value.
 struct ThermalBoundaryConditionDefinition {
   EntityIndex node_index;