refactor(srp): move shadow caster stage selection into managed urp

- add standalone pass asset factories for camera frame stages\n- let managed pipeline assets declare stage pass asset keys\n- make universal renderer data explicitly own the builtin shadow caster stage

managed/XCEngine.RenderPipelines.Universal/Rendering/Universal/UniversalRendererData.cs

@@ -32,6 +32,20 @@ namespace XCEngine.Rendering.Universal
             return "BuiltinForward";
         }
 
+        protected override string GetCameraFrameStandalonePassAssetKey(
+            CameraFrameStage stage)
+        {
+            switch (stage)
+            {
+                case CameraFrameStage.ShadowCaster:
+                    return "BuiltinShadowCaster";
+                default:
+                    return base
+                        .GetCameraFrameStandalonePassAssetKey(
+                            stage);
+            }
+        }
+
         internal UniversalMainSceneData GetMainSceneInstance()
         {
             if (mainScene == null)