refactor(srp): remove managed frame planning hook

- derive fullscreen stage planning from pipeline render-graph support
- trim planning-only APIs from the managed SRP bridge and public surface
- update probes and tests to lock the slimmer SRP API boundary

managed/XCEngine.ScriptCore/InternalCalls.cs

@@ -700,19 +700,6 @@ namespace XCEngine
             Rendering_ScriptableRenderPipelineCameraRequestContext_ClearDirectionalShadow(
                 ulong nativeHandle);
 
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        internal static extern void
-            Rendering_ScriptableRenderPipelinePlanningContext_ClearFullscreenStage(
-                ulong nativeHandle,
-                int stage);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        internal static extern bool
-            Rendering_ScriptableRenderPipelinePlanningContext_RequestFullscreenStage(
-                ulong nativeHandle,
-                int stage,
-                int source,
-                bool usesGraphManagedOutputColor);
     }
 }