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/ScriptableRendererData.cs

@@ -61,6 +61,13 @@ namespace XCEngine.Rendering.Universal
             return GetPipelineRendererAssetKey();
         }
 
+        internal string GetCameraFrameStandalonePassAssetKeyInstance(
+            CameraFrameStage stage)
+        {
+            return GetCameraFrameStandalonePassAssetKey(
+                stage);
+        }
+
         internal int GetRuntimeStateVersionInstance()
         {
             return m_runtimeStateVersion;
@@ -153,6 +160,12 @@ namespace XCEngine.Rendering.Universal
             return string.Empty;
         }
 
+        protected virtual string GetCameraFrameStandalonePassAssetKey(
+            CameraFrameStage stage)
+        {
+            return string.Empty;
+        }
+
         protected virtual ScriptableRendererFeature[] CreateRendererFeatures()
         {
             return Array.Empty<ScriptableRendererFeature>();