refactor(srp): bridge universal shadow planning settings into managed asset

- expose directional shadow planning settings on camera request context\n- let the managed universal asset override planner defaults\n- recompute native directional shadow requests only when settings change

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

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+# SRP Universal Shadow Planning Settings Bridge Plan 2026-04-21
+
+## Goal
+
+Extend the current managed shadow ownership from a simple enable/disable switch to a real planning-settings bridge:
+
+1. managed can read the current directional shadow planning settings;
+2. managed can override those settings;
+3. native recomputes the shadow request after managed overrides, without changing the existing call order.
+
+## Why This Stage
+
+The previous stage moved the first shadow ownership decision into `UniversalRenderPipelineAsset`, but it still only controlled whether shadows stay enabled.
+
+The remaining gap is that native already receives `DirectionalShadowPlanningSettings`, while the managed runtime currently discards them. Until that bridge exists, URP cannot meaningfully own shadow planning defaults.
+
+## Scope
+
+Included:
+
+1. Add managed mirror structs for directional shadow planning settings.
+2. Expose those settings on `CameraRenderRequestContext` and `RendererCameraRequestContext`.
+3. Extend the Mono bridge state to carry a mutable settings copy and a dirty flag.
+4. Re-run native default shadow planning after managed request configuration when settings changed.
+5. Let `UniversalShadowSettings` push its planning defaults into the request context.
+6. Rebuild `XCEditor` and run old editor smoke.
+
+Not included:
+
+1. Shadow cascades.
+2. Shadow atlas / multiple lights.
+3. C# shadow map raster pass implementation.
+4. Editor UI for shadow settings.
+
+## Acceptance
+
+This stage is complete when:
+
+1. managed URP can override planner-owned main directional shadow defaults;
+2. existing `ClearDirectionalShadow()` behavior still works;
+3. the native shadow request is recomputed only when managed changed planning settings;
+4. `XCEditor` build and old editor smoke both pass.