feat: 实现 D3D12DescriptorHeap 描述符堆类

- 添加 D3D12DescriptorHeap.h 头文件
- 实现 ID3D12DescriptorHeap 封装
- 支持 RTV、DSV、CBV_SRV_UAV、Sampler 堆类型
- 支持 GPU 可见描述符堆
- 添加 GetCPUDescriptorHandle、GetGPUDescriptorHandle 等方法
- 测试通过

engine/include/XCEngine/RHI/D3D12/D3D12DescriptorHeap.h

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <d3d12.h>
+#include <wrl/client.h>
+
+#include "../Enums.h"
+#include "D3D12Enum.h"
+
+using Microsoft::WRL::ComPtr;
+
+namespace XCEngine {
+namespace RHI {
+
+class D3D12DescriptorHeap {
+public:
+    D3D12DescriptorHeap();
+    ~D3D12DescriptorHeap();
+
+    bool Initialize(ID3D12Device* device, DescriptorHeapType type, uint32_t numDescriptors, bool shaderVisible = false);
+    void Shutdown();
+
+    ID3D12DescriptorHeap* GetDescriptorHeap() const { return m_descriptorHeap.Get(); }
+
+    D3D12_CPU_DESCRIPTOR_HANDLE GetCPUDescriptorHandle(uint32_t index) const;
+    D3D12_GPU_DESCRIPTOR_HANDLE GetGPUDescriptorHandle(uint32_t index) const;
+
+    uint32_t GetDescriptorCount() const { return m_numDescriptors; }
+    DescriptorHeapType GetType() const { return m_type; }
+    uint32_t GetDescriptorSize() const { return m_descriptorSize; }
+
+    D3D12_CPU_DESCRIPTOR_HANDLE GetCPUDescriptorHandleForHeapStart() const;
+    D3D12_GPU_DESCRIPTOR_HANDLE GetGPUDescriptorHandleForHeapStart() const;
+
+private:
+    ComPtr<ID3D12DescriptorHeap> m_descriptorHeap;
+    DescriptorHeapType m_type;
+    uint32_t m_numDescriptors;
+    uint32_t m_descriptorSize;
+    bool m_shaderVisible;
+};
+
+} // namespace RHI
+} // namespace XCEngine