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/Engine.vcxproj.filters

@@ -25,6 +25,10 @@
     <ClInclude Include="EngineAPI\Camera.h" />
     <ClInclude Include="EngineAPI\Light.h" />
     <ClInclude Include="Graphics\Renderer.h" />
+    <ClInclude Include="Graphics\Direct3D12\D3D12CommonHeader.h" />
+    <ClInclude Include="Graphics\Direct3D12\D3D12Core.h" />
+    <ClInclude Include="Graphics\Direct3D12\D3D12Interface.h" />
+    <ClInclude Include="Graphics\GraphicsPlatformInterface.h" />
   </ItemGroup>
   <ItemGroup>
     <ClCompile Include="Components\Entity.cpp" />
@@ -37,5 +41,8 @@
     <ClCompile Include="Platform\PlatformWin32.cpp" />
     <ClCompile Include="Platform\Window.cpp" />
     <ClCompile Include="Content\ContentToEngine.cpp" />
+    <ClCompile Include="Graphics\Renderer.cpp" />
+    <ClCompile Include="Graphics\Direct3D12\D3D12Core.cpp" />
+    <ClCompile Include="Graphics\Direct3D12\D3D12Interface.cpp" />
   </ItemGroup>
 </Project>