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

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

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+# SRP Universal Shadow Caster Stage Managed Ownership Plan 2026-04-21
+
+## Goal
+
+Move `CameraFrameStage::ShadowCaster` pass selection out of the native host hardcode and into managed URP ownership.
+
+This stage does **not** move shadow rasterization itself into C#.
+It only makes managed URP explicitly declare which native shadow-caster pass asset it wants to use.
+
+## Why This Stage
+
+The current managed URP already owns:
+
+1. renderer selection;
+2. main-scene builtin feature ownership;
+3. shadow enable / planning defaults.
+
+But the actual `ShadowCaster` standalone stage is still installed in native host code through a hardcoded `BuiltinShadowCasterPass`.
+
+That means shadow stage ownership is still not aligned with the SRP/URP boundary.
+
+## Scope
+
+Included:
+
+1. add a native registry/factory for camera-frame standalone pass assets;
+2. let managed `ScriptableRenderPipelineAsset` expose standalone pass asset keys by stage;
+3. let `RendererBackedRenderPipelineAsset` route that decision through selected renderer data;
+4. make `UniversalRendererData` explicitly declare the builtin shadow-caster pass asset key;
+5. let `ScriptableRenderPipelineHost` resolve contextual standalone passes from managed asset runtime instead of hardcoding shadow caster;
+6. rebuild `XCEditor` and run old editor smoke.
+
+Not included:
+
+1. managed shadow raster pass recording;
+2. cascades;
+3. shadow atlas packing;
+4. editor UI for standalone pass selection.
+
+## Acceptance
+
+This stage is complete when:
+
+1. managed URP explicitly owns the `ShadowCaster` standalone pass selection;
+2. native host no longer hardcodes shadow caster for managed pipelines;
+3. builtin fallback pipeline still keeps depth-only / object-id / shadow-caster working;
+4. `XCEditor` build and old editor smoke both pass.