Move prepared-file work into app runtime

docs/modernization/tasks.md

@@ -146,6 +146,9 @@ Current slice:
 - `NodeCanvas` heightmap draw callback setup now also routes through
   `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.
 - `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
@@ -347,6 +350,8 @@ Current slice:
 - `main.cpp` Win32 window handles, GL task/mutex state, splash-dialog state,
   stylus timers, and VR worker state now sit behind one retained local state
   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`
 - retained `App` composition, task call sites, and platform/runtime entrypoint
   coupling are still not fully reduced behind the runtime contract
 
@@ -404,12 +409,16 @@ 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
 - 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
+  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
 
 Write scope:
 - `src/canvas.cpp`