feat: 实现 D3D12RootSignature 根签名类

- 添加 D3D12RootSignature.h 头文件
- 实现 ID3D12RootSignature 封装
- 支持序列化根签名描述符
- 测试通过

engine/include/XCEngine/RHI/D3D12/D3D12RootSignature.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <d3d12.h>
+#include <wrl/client.h>
+
+#include "../Enums.h"
+#include "D3D12Enum.h"
+
+using Microsoft::WRL::ComPtr;
+
+namespace XCEngine {
+namespace RHI {
+
+class D3D12RootSignature {
+public:
+    D3D12RootSignature();
+    ~D3D12RootSignature();
+
+    bool Initialize(ID3D12Device* device, const D3D12_ROOT_SIGNATURE_DESC& desc);
+    void Shutdown();
+
+    ID3D12RootSignature* GetRootSignature() const { return m_rootSignature.Get(); }
+
+private:
+    ComPtr<ID3D12RootSignature> m_rootSignature;
+};
+
+} // namespace RHI
+} // namespace XCEngine