D3D12: Implement ClearRenderTarget by tracking bound RTV handles

- Add m_boundRenderTargets and m_boundDepthStencil members to D3D12CommandList
- SetRenderTargetsHandle now saves bound RTV handles for later clear operations
- ClearRenderTargetView(ID3D12Resource*, ...) clears all bound render targets
- Reset() clears bound targets list
- Follows D3D12 best practice: RTV is bound via OMSetRenderTargets, then cleared

engine/include/XCEngine/RHI/D3D12/D3D12CommandList.h

@@ -118,6 +118,10 @@ private:
     ID3D12PipelineState* m_currentPipelineState;
     ID3D12RootSignature* m_currentRootSignature;
     ID3D12DescriptorHeap* m_currentDescriptorHeap;
+
+    std::vector<D3D12_CPU_DESCRIPTOR_HANDLE> m_boundRenderTargets;
+    D3D12_CPU_DESCRIPTOR_HANDLE m_boundDepthStencil;
+    bool m_depthStencilBound = false;
 };
 
 } // namespace RHI