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/GraphicsPlatformInterface.h

@@ -0,0 +1,11 @@
+#pragma once    
+#include "CommonHeader.h"
+#include "Renderer.h"
+
+namespace XEngine::graphics{
+struct platform_interface{
+    bool(*initialize)(void);
+    void(*shutdown)(void);
+};
+}// namespace XEngine::graphics
+