feat: add Graphics module with D3D12 backend and documentation

Graphics Module:
- Add platform abstraction layer (GraphicsPlatformInterface)
- Add unified renderer entry point (Renderer)
- Add D3D12 backend implementation (D3D12Core, D3D12Interface)
- Add TestRenderer for multi-window rendering tests

Documentation:
- Add Graphics渲染架构分析.md
- Add D3D12学习Wiki.md
- Add changelogs directory with per-commit documentation
- Add 20260326-dx12-initial.md for initial framework
- Add 20260326-d3d12-foundation.md for Graphics module

Fixes:
- Resolve header include issues and type redefinition errors

Engine/Graphics/Direct3D12/D3D12Core.cpp

@@ -0,0 +1,20 @@
+#include "D3D12Core.h"
+#include "D3D12CommonHeader.h"
+
+namespace XEngine::graphics::d3d12::core{
+namespace{
+
+ID3D12Device8* main_device;
+
+}// anonymous namespace
+bool 
+initialize()
+{
+    // determine which adapter
+	return true;
+}
+void 
+shutdown()
+{
+}
+}// namespace XEngine::graphics::d3d12::core