Route app async work through AppRuntime

docs/modernization/tasks.md

@@ -59,6 +59,12 @@ Key facts:
 - `AppRuntime` now owns synchronized running flags plus explicit post/reject,
   same-thread execution, and queue-drain behavior, but broader singleton reach
   and app-shell ownership remain.
+- Retained cloud upload/download, brush-package import, and timelapse-export
+  async paths now route through `AppRuntime::canvas_async_task`, but dialog and
+  execution ownership still remains in retained app/document/cloud bridges.
+- `App::dialog_browse()` no longer owns browse-dialog button wiring inline; the
+  retained document-open bridge now owns that handoff in
+  `src/legacy_document_open_services.*`.
 
 ## Parallel Assignment Rules