Harden app runtime and thin export shell

docs/modernization/debt.md

@@ -18,6 +18,23 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Reductions
 
+- 2026-06-17: `DEBT-0003` was narrowed again. `src/app_runtime.cpp` and
+  `src/app_runtime.h` now synchronize render/UI worker running state, reject
+  cross-thread work once those workers stop, and drain queued work through
+  explicit runtime-owned post/shutdown semantics consumed by
+  `src/legacy_app_runtime_shell_services.cpp`; broader singleton reach, app
+  shell ownership, and remaining runtime service adoption still remain.
+- 2026-06-17: `DEBT-0017` was narrowed again.
+  `cmake/PanoPainterSources.cmake` no longer compiles
+  `${PP_PLATFORM_WEB_SOURCES}` into `PP_PANOPAINTER_APP_SOURCES`, so root app
+  source ownership no longer mixes live Web platform implementation files into
+  the retained app group; broader platform entrypoint/package cleanup remains.
+- 2026-06-17: `DEBT-0043` was narrowed again.
+  `src/legacy_canvas_document_io_services.cpp` now delegates the live
+  equirectangular export family through `src/legacy_document_export_services.*`
+  instead of carrying that orchestration inline with the broader canvas
+  document-I/O shell; retained export directory creation, Web handoff, and
+  remaining legacy `Canvas` export execution still remain.
 - 2026-06-17: `DEBT-0003` was narrowed again.
   `src/platform_windows/windows_runtime_flow.*` now owns the live Win32
   startup/session composition flow, including the bound app/session preflight