fix: set default DepthStencilState in graphics_device_init

Prevents UILayer init from breaking 3D rendering pipeline.
Creates explicit LESS_EQUAL depth state at init time.

Engine/Rendering/graphics_device.c

@@ -65,6 +65,18 @@ int graphics_device_init(GraphicsDevice* d, void* hwnd, uint32_t w, uint32_t h,
     vp.Width = (float)w; vp.Height = (float)h;
     vp.MaxDepth = 1.0f;
     d->context->lpVtbl->RSSetViewports(d->context, 1, &vp);
+
+    // 设置默认深度状态 (避免 UILayer init 破坏)
+    D3D11_DEPTH_STENCIL_DESC dsd = {0};
+    dsd.DepthEnable = TRUE;
+    dsd.DepthWriteMask = D3D11_DEPTH_WRITE_MASK_ALL;
+    dsd.DepthFunc = D3D11_COMPARISON_LESS_EQUAL;
+    ID3D11DepthStencilState* default_dss = NULL;
+    d->device->lpVtbl->CreateDepthStencilState(d->device, &dsd, &default_dss);
+    if (default_dss) {
+        d->context->lpVtbl->OMSetDepthStencilState(d->context, default_dss, 0);
+        default_dss->lpVtbl->Release(default_dss);
+    }
     return 1;
 }