fix(D3D12): 修复 D3D12Fence::IsSignaled() 实现错误

- 原来实现假设 GetCompletedValue() > 0 即为 signaled，这是错误的
- 添加 m_signalValue 成员变量跟踪最后一次 signal 的值
- IsSignaled() 现在正确检查 GetCompletedValue() >= m_signalValue

engine/include/XCEngine/RHI/D3D12/D3D12Fence.h

@@ -22,7 +22,7 @@ public:
     void Signal(uint64_t value) override;
     void Wait(uint64_t value) override;
     uint64_t GetCompletedValue() const override;
-    bool IsSignaled() const override { return GetCompletedValue() > 0; }
+    bool IsSignaled() const override { return m_fence->GetCompletedValue() >= m_signalValue; }
     void* GetEventHandle() { return m_eventHandle; }
 
     void* GetNativeHandle() override { return m_fence.Get(); }
@@ -31,6 +31,7 @@ public:
 private:
     ComPtr<ID3D12Fence> m_fence;
     void* m_eventHandle;
+    uint64_t m_signalValue = 0;
 };
 
 } // namespace RHI