feat: implement descriptor heap with thread-safe allocation

D3D12 Resources:
- Add descriptor_handle struct with CPU/GPU handles
- Add descriptor_heap class for descriptor management
- Implement allocate() and free() methods
- Add mutex for thread-safe access
- Support all D3D12 descriptor heap types

D3D12 Core:
- Add device() function to expose main device
- Add release() template function for COM objects

Documentation:
- Add changelog for descriptor heap implementation
- Update D3D12 Wiki with descriptor heap section
- Mark descriptor heap task as completed

Engine/Graphics/Direct3D12/D3D12Core.cpp

@@ -421,4 +421,11 @@ render()
 	// 为下一帧标记并增加围栏值
 	gfx_command.end_frame();
 }
+
+ID3D12Device *const 
+device()
+{
+	return main_device;
+}
+
 }// namespace XEngine::graphics::d3d12::core