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/Renderer.h

@@ -0,0 +1,28 @@
+#pragma once
+#include "CommonHeader.h"
+#include "..\Platform\Window.h"
+
+
+namespace XEngine::graphics {
+
+class surface
+{
+};
+
+struct render_surface
+{
+    platform::window window{};
+    surface surface{};
+};
+
+enum class graphics_platform : u32
+{
+    direct3d12 = 0,
+    vulkan = 1,
+    opengl = 2,
+};
+
+bool initialize(graphics_platform platform);    
+void shutdown();
+
+}