feat: integrate Solid source and three-DOF semantics

include/fesa/elements/element_definition.h

@@ -9,7 +9,12 @@
 namespace fesa {
 
 /// @brief Identifies the supported semantic element-definition kinds.
-enum class ElementDefinitionKind { kEulerBeam3D, kMitc4Shell, kDs4Thermal };
+enum class ElementDefinitionKind {
+  kEulerBeam3D,
+  kMitc4Shell,
+  kDs4Thermal,
+  kHex8
+};
 
 /// @brief Provides immutable source identity and topology for one element.
 /// @note Numerical stiffness, recovery, and equation ids are intentionally

include/fesa/elements/hex8_definition.h

@@ -0,0 +1,58 @@
+#ifndef FESA_ELEMENTS_HEX8_DEFINITION_H_
+#define FESA_ELEMENTS_HEX8_DEFINITION_H_
+
+#include <array>
+#include <utility>
+#include <vector>
+
+#include "fesa/elements/element_definition.h"
+
+namespace fesa {
+/// @brief Owns source-safe eight-node Solid topology without numerical state.
+class Hex8Definition final : public ElementDefinition {
+ public:
+  /// @brief Constructs a semantic candidate in source connectivity order.
+  Hex8Definition(SourceEntityId source, std::array<EntityIndex, 8> nodes,
+                 EntityIndex material, EntityIndex section,
+                 SourceLocation source_location)
+      : source_id{std::move(source)},
+        node_indices{nodes},
+        material_index{material},
+        section_index{section},
+        location{std::move(source_location)},
+        property_index_{section},
+        node_indices_view_{nodes.begin(), nodes.end()} {}
+  ElementDefinitionKind Kind() const noexcept override {
+    return ElementDefinitionKind::kHex8;
+  }
+  const SourceEntityId& SourceId() const noexcept override { return source_id; }
+  std::string_view SourceElementType() const noexcept override {
+    return "C3D8";
+  }
+  const std::vector<EntityIndex>& NodeIndices() const noexcept override {
+    return node_indices_view_;
+  }
+  EntityIndex MaterialIndex() const noexcept override { return material_index; }
+  EntityIndex PropertyIndex() const noexcept override {
+    return property_index_;
+  }
+
+  SourceEntityId source_id;
+  std::array<EntityIndex, 8> node_indices;
+  EntityIndex material_index;
+  EntityIndex section_index;
+  SourceLocation location;
+
+ private:
+  friend class Domain;
+  /// @brief Synchronizes candidate topology with the immutable base view.
+  void SynchronizeNodeIndices() {
+    node_indices_view_.assign(node_indices.begin(), node_indices.end());
+  }
+  /// @brief Binds the local Solid section to the unified property collection.
+  void SetPropertyIndex(EntityIndex index) noexcept { property_index_ = index; }
+  EntityIndex property_index_;
+  std::vector<EntityIndex> node_indices_view_;
+};
+}  // namespace fesa
+#endif  // FESA_ELEMENTS_HEX8_DEFINITION_H_