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/D3D12Interface.cpp

@@ -0,0 +1,14 @@
+#include "D3D12Interface.h"
+#include "D3D12Core.h"
+#include "Graphics\GraphicsPlatformInterface.h"
+
+namespace XEngine::graphics::d3d12{
+void get_platform_interface(platform_interface& pi)
+{
+	pi.initialize = core::initialize;
+    pi.shutdown = core::shutdown;
+}
+}// namespace XEngine::graphics::d3d12
+
+
+