Route app shutdown staging through app core

docs/modernization/debt.md

@@ -20,7 +20,7 @@ agent or engineer to remove them without reconstructing context from chat.
 
 - 2026-06-04: DEBT-0009 was narrowed. `platform-build.ps1` and
   `platform-build.sh` now include the current headless component/test matrix,
-  including brush-package coverage and the app-core startup/frame/file/document/
+  including brush-package coverage and the app-core startup/frame/shutdown/file/document/
   brush/canvas/history/grid/toolbar/tools/about/preferences/status automation
   tests, and `panopainter_platform_build_target_matrix_self_test` now verifies
   the wrapper defaults against the current CMake test executables. On
@@ -97,6 +97,13 @@ agent or engineer to remove them without reconstructing context from chat.
   consumed by `App::create`, `App::update`, `App::draw`, and
   `pano_cli plan-app-frame`; retained layout traversal, toolbar widget writes,
   and OpenGL/UI drawing remain in the legacy app.
+- 2026-06-05: DEBT-0003 was narrowed again. Shutdown cleanup staging for
+  UI-state save, stroke-preview renderer shutdown, recording stop,
+  texture/shader invalidation, layout unload, UI render-target and face-plane
+  destruction, panel-node release, and quick-mode cleanup now goes through
+  tested `pp_app_core` plans consumed by `App::terminate` and
+  `pano_cli plan-app-shutdown`; retained cleanup execution remains in the
+  legacy app.
 - 2026-06-04: DEBT-0036 was narrowed again. Canvas stroke commit,
   thumbnail, and object-draw history paths now query saved blend state through
   tested `pp_renderer_gl` capability-state dispatch; CanvasLayer equirect