refactor(srp): remove managed backend key binding detour

docs/used/SRP_URP_DefaultNativeRendererBindingPlan_完成归档_2026-04-21.md

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+# SRP URP Default Native Renderer Binding Plan
+
+Date: 2026-04-21
+
+## Goal
+
+Remove the remaining managed backend-key declaration path from current URP
+renderer data and make native scene-renderer binding fall back to an internal
+default native renderer asset instead of asking managed C# for a string key.
+
+This keeps the native renderer substrate in C++, but stops routing current URP
+through a Unity-unlike `"BuiltinForward"` key callback.
+
+## Why This Stage
+
+After the previous SRP cleanup:
+
+1. managed standalone-pass asset keys are gone;
+2. managed URP owns camera request, frame planning, render-scene setup,
+   shadow/depth stage composition, and stage recording;
+3. the remaining managed/native backend seam is
+   `GetPipelineRendererAssetKey*()`.
+
+Current repo state shows:
+
+1. `UniversalRendererData` still returns `"BuiltinForward"` from managed C#;
+2. `MonoManagedRenderPipelineAssetRuntime` still resolves that method name and
+   converts the string key back into a native asset;
+3. the only active builtin backend is still the default forward renderer
+   substrate.
+
+So the current C# backend-key hop is not buying real flexibility. It is mostly
+an internal detour that keeps Unity-unlike ownership in the current URP path.
+
+## Scope
+
+Included:
+
+1. add an internal helper that resolves the default native pipeline renderer
+   asset in C++;
+2. remove `GetPipelineRendererAssetKey*()` from current managed URP C# types;
+3. remove the corresponding Mono method-resolution / invocation bridge;
+4. make `MonoManagedRenderPipelineAssetRuntime::GetPipelineRendererAsset(...)`
+   use the internal default native renderer asset;
+5. rebuild `XCEditor` and run old editor smoke.
+
+Not included:
+
+1. redesigning `NativeSceneRecorder` itself;
+2. introducing deferred native scene renderer backends;
+3. changing public SRP authoring APIs;
+4. moving scene draw implementation out of C++.
+
+## Done Criteria
+
+1. current URP managed code no longer returns native backend keys;
+2. Mono runtime no longer resolves `GetPipelineRendererAssetKey*()` from
+   managed assets;
+3. managed SRP host / stage recorder still binds a valid native scene renderer
+   substrate through internal default binding;
+4. `cmake --build . --config Debug --target XCEditor` passes;
+5. old editor smoke passes for at least about 10 seconds and a fresh
+   `SceneReady` appears in `editor/bin/Debug/editor.log`.