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.cpp

@@ -0,0 +1,38 @@
+#include "Renderer.h"
+#include "GraphicsPlatformInterface.h"
+#include "Direct3D12/D3D12Interface.h"
+
+namespace XEngine::graphics {
+namespace {
+platform_interface gfx{};
+bool
+set_platform_interface(graphics_platform platform)
+{
+	switch (platform)
+	{
+	case graphics_platform::direct3d12:
+		d3d12::get_platform_interface(gfx);
+		break;
+	case graphics_platform::vulkan:
+		//vulkan::get_platform_interface(gfx);
+		break;
+	case graphics_platform::opengl:
+		//opengl::get_platform_interface(gfx);
+		break;
+	default:
+		return false;
+	}
+	return gfx.initialize();
+}
+}// anonymous namespace
+
+bool initialize(graphics_platform platform)
+{
+	return set_platform_interface(platform);
+}
+
+void shutdown()
+{
+	gfx.shutdown();	
+}
+}// namespace XEngine::graphics