refactor: reorganize Resources module into Core/Asset, Core/IO and Resources subdirectories

- Split core resource architecture into Core/Asset/ (IResource, ResourceManager, ResourceCache, etc.)
- Moved IO layer into Core/IO/ (IResourceLoader, ResourceFileSystem, etc.)
- Reorganized concrete resource types into Resources/{Texture,Mesh,Material,Shader,AudioClip}/
- Updated all include paths from relative to <XCEngine/...> format
- Fixed circular dependency in Material.h (removed unnecessary ResourceManager.h include)
- Fixed malformed include syntax in ResourceManager.h and AsyncLoader.h
- Fixed glad.h path issues in CMakeLists.txt

engine/src/Resources/Mesh/Mesh.cpp

@@ -0,0 +1,44 @@
+#include "Resources/Mesh.h"
+#include <cstring>
+
+namespace XCEngine {
+namespace Resources {
+
+Mesh::Mesh() = default;
+Mesh::~Mesh() = default;
+
+void Mesh::Release() {
+    m_vertexData.Clear();
+    m_indexData.Clear();
+    m_sections.Clear();
+    SetInvalid();
+}
+
+void Mesh::SetVertexData(const void* data, size_t size, Core::uint32 vertexCount,
+                        Core::uint32 vertexStride, VertexAttribute attributes) {
+    m_vertexCount = vertexCount;
+    m_vertexStride = vertexStride;
+    m_attributes = attributes;
+    
+    m_vertexData.Resize(size);
+    std::memcpy(m_vertexData.Data(), data, size);
+    
+    m_memorySize += size;
+}
+
+void Mesh::SetIndexData(const void* data, size_t size, Core::uint32 indexCount, bool use32Bit) {
+    m_indexCount = indexCount;
+    m_use32BitIndex = use32Bit;
+    
+    m_indexData.Resize(size);
+    std::memcpy(m_indexData.Data(), data, size);
+    
+    m_memorySize += size;
+}
+
+void Mesh::AddSection(const MeshSection& section) {
+    m_sections.PushBack(section);
+}
+
+} // namespace Resources
+} // namespace XCEngine