Move canvas async work into app runtime

docs/modernization/tasks.md

@@ -147,8 +147,10 @@ Current slice:
   `make_legacy_canvas_draw_merge_heightmap_draw(...)`, but the node still owns
   current-mode traversal and broader post-draw orchestration.
 - `NodeCanvas` current-mode draw callback setup now also routes through
-  `make_legacy_canvas_draw_merge_current_modes_draw(...)`, but grid-mode
-  traversal and broader post-draw orchestration are still inline.
+  `make_legacy_canvas_draw_merge_current_modes_draw(...)`, and grid-mode
+  callback setup now also routes through
+  `make_legacy_canvas_draw_merge_grid_modes_draw(...)`, but broader post-draw
+  orchestration is still inline.
 - `NodeCanvas` smoothing-mask face shader setup plus per-face draw execution
   now also route through
   `execute_legacy_canvas_draw_merge_smask_faces(...)`, but the node still owns
@@ -352,6 +354,9 @@ Current slice:
   object instead of separate file-scope globals
 - prepared-file background work now runs through an `AppRuntime`-owned worker
   queue instead of a retained static worker in `src/app_events.cpp`
+- canvas async import/export/save/open background work now also runs through an
+  `AppRuntime`-owned worker queue instead of a retained static worker in
+  `src/canvas.cpp`
 - retained `App` composition, task call sites, and platform/runtime entrypoint
   coupling are still not fully reduced behind the runtime contract
 
@@ -409,16 +414,18 @@ Current slice:
 - `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
-- the prepared-file worker has now moved again into `AppRuntime`, removing the
-  retained static worker from `src/app_events.cpp`; the broader canvas async
-  worker still remains local because that slice is wider
+- the prepared-file worker and the broader canvas async import/export/save/open
+  worker have now both moved into `AppRuntime`, removing the retained static
+  workers from `src/app_events.cpp` and `src/canvas.cpp`
 - 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, its iteration-context setup now also routes 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
+  and its wait/plan/encode iteration shell now also routes through a local
+  helper, while `App::update()` no longer carries the dead update mutex
+  residue; retained recording loop control and readback ownership are still
+  open
 
 Write scope:
 - `src/canvas.cpp`