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

include/fesa/properties/element_property.h

@@ -6,7 +6,11 @@
 namespace fesa {
 
 /// @brief Identifies the supported concrete element-property semantics.
-enum class ElementPropertyKind { kGeneralBeamSection, kShellSection };
+enum class ElementPropertyKind {
+  kGeneralBeamSection,
+  kShellSection,
+  kSolidSection
+};
 
 /// @brief Provides stable identity for a Domain-owned element property.
 class ElementProperty {

include/fesa/properties/solid_section.h

@@ -0,0 +1,39 @@
+#ifndef FESA_PROPERTIES_SOLID_SECTION_H_
+#define FESA_PROPERTIES_SOLID_SECTION_H_
+
+#include <string>
+#include <utility>
+
+#include "fesa/properties/element_property.h"
+
+namespace fesa {
+/// @brief Owns a homogeneous Solid material assignment without thickness.
+class SolidSection final : public ElementProperty {
+ public:
+  /// @brief Constructs a parser-validated ELSET and material assignment.
+  SolidSection(SourceEntityId source, std::string name,
+               EntityIndex material_index, SourceLocation location)
+      : source_id_{std::move(source)},
+        name_{std::move(name)},
+        material_index_{material_index},
+        location_{std::move(location)} {}
+  ElementPropertyKind Kind() const noexcept override {
+    return ElementPropertyKind::kSolidSection;
+  }
+  const SourceEntityId& SourceId() const noexcept override {
+    return source_id_;
+  }
+  const SourceLocation& Location() const noexcept override { return location_; }
+  /// @brief Returns the source ELSET spelling.
+  const std::string& Name() const noexcept { return name_; }
+  /// @brief Returns the stable Domain material index.
+  EntityIndex MaterialIndex() const noexcept { return material_index_; }
+
+ private:
+  SourceEntityId source_id_;
+  std::string name_;
+  EntityIndex material_index_;
+  SourceLocation location_;
+};
+}  // namespace fesa
+#endif  // FESA_PROPERTIES_SOLID_SECTION_H_