Trim recording loop and retain async worker state

docs/modernization/tasks.md

@@ -401,9 +401,15 @@ Current slice:
   workers with explicit UI-thread handoff
 - canvas async import/export/save/open and timelapse export now also use owned
   worker queues instead of detached threads
+- `src/app_events.cpp` prepared-file worker ownership and `src/canvas.cpp`
+  async import/export/save/open worker ownership now also sit behind named
+  retained local worker-state helpers instead of bare static worker accessors
 - preview background rendering, recording, and the retained
   `NodePanelGrid::bake_uvs()` worker now also use `std::jthread`, but their
   retained loop/control flow is still open
+- `App::rec_loop()` now routes its frame encode/update chunk through a local
+  helper, and `App::update()` no longer carries the dead update mutex residue,
+  but retained recording loop control and readback ownership are still open
 
 Write scope:
 - `src/canvas.cpp`