Route app shutdown staging through app core

docs/modernization/build-inventory.md

@@ -246,6 +246,11 @@ Known local toolchain state:
   `App::draw`, and `pano_cli plan-app-frame`; retained layout traversal,
   toolbar widget writes, canvas stroke drawing, VR UI render-target drawing,
   main target binding, and OpenGL/UI drawing remain in the legacy app.
+- `src/app_core/app_shutdown.h` owns the current shutdown cleanup staging
+  consumed by `App::terminate` and `pano_cli plan-app-shutdown`; retained
+  UI-state save, recording stop, resource invalidation, layout unload,
+  render-target/mesh destruction, and panel-node release remain in the legacy
+  app.
 - `src/legacy_app_preference_services.*` is the current app-shell bridge for
   options-menu preference execution. It keeps UI scale, viewport scale, RTL,
   VR mode, VR-controller, auto-timelapse, and canvas cursor-mode callbacks on
@@ -832,6 +837,10 @@ Known local toolchain state:
 - `pp_app_core_app_frame_tests` covers the legacy initial surface default,
   idle/redraw/animation update gating, canvas-stroke draw eligibility, VR UI
   visibility, main UI suppression in VR-only mode, and redraw reset planning.
+- `pp_app_core_app_shutdown_tests` covers legacy shutdown cleanup staging for
+  UI-state save, stroke-preview renderer shutdown, recording stop,
+  texture/shader invalidation, layout unload, render-target/mesh destruction,
+  panel-node release, and quick-mode cleanup.
 - `pp_platform_api_tests` covers service dispatch for clipboard read/write,
   empty clipboard writes, cursor visibility, virtual-keyboard visibility,
   external file display, file sharing, VR lifecycle, layout/asset file load