feat: 实现D3D12 RHI抽象层，修复PSO创建问题

- 添加RHI接口定义(IRHIDevice, ICommandList, IResource等)
- 实现D3D12Device, D3D12CommandList, D3D12PipelineState等
- 修复RootSignature参数数量(3->4)与HelloEarth一致
- 修复DSV格式设置(Unknown->D24_UNorm_S8_UInt)
- 添加Geometry Shader编译
- 创建XCEngineDemo项目验证RHI功能

engine/src/RHI/D3D12/D3D12Shader.cpp

@@ -0,0 +1,40 @@
+#include <Rendering\Shader.h>
+#include <d3dcompiler.h>
+#include <wrl/client.h>
+#include <string>
+
+namespace XCEngine {
+namespace RHI {
+
+bool CompileShader(const char* filePath, const char* entryPoint, const char* target, ShaderBytecode& outBytecode) {
+    UINT flags = D3DCOMPILE_DEBUG | D3DCOMPILE_SKIP_OPTIMIZATION;
+    
+    ID3DBlob* shaderBuffer = nullptr;
+    ID3DBlob* errorBuffer = nullptr;
+    
+    HRESULT hr = D3DCompileFromFile(
+        std::wstring(filePath, filePath + strlen(filePath)).c_str(),
+        nullptr,
+        nullptr,
+        entryPoint,
+        target,
+        flags,
+        0,
+        &shaderBuffer,
+        &errorBuffer);
+    
+    if (FAILED(hr)) {
+        if (errorBuffer) {
+            OutputDebugStringA((const char*)errorBuffer->GetBufferPointer());
+            errorBuffer->Release();
+        }
+        return false;
+    }
+    
+    outBytecode.bytecode = shaderBuffer->GetBufferPointer();
+    outBytecode.size = shaderBuffer->GetBufferSize();
+    return true;
+}
+
+} // namespace RHI
+} // namespace XCEngine