Move canvas async work into app runtime

docs/modernization/debt.md

@@ -18,6 +18,21 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Reductions
 
+- 2026-06-16: `DEBT-0003` was narrowed again. Canvas async
+  import/export/save/open background work now runs through an
+  `AppRuntime`-owned queue/worker in `src/app_runtime.h/.cpp`, and
+  `src/canvas.cpp` no longer defines a retained static canvas async worker;
+  broader app task ownership and retained runtime singleton reach remain.
+- 2026-06-16: `DEBT-0037` was narrowed again. `App::rec_loop()` in
+  `src/app.cpp` now routes its wait plus iteration plan/encode shell through a
+  local helper instead of carrying that orchestration inline; retained
+  recording loop control, readback call sites, and MP4 execution remain.
+- 2026-06-16: `DEBT-0036` was narrowed again. `NodeCanvas` grid-mode draw
+  callback setup now routes through
+  `make_legacy_canvas_draw_merge_grid_modes_draw(...)` in
+  `src/legacy_canvas_draw_merge_services.h` instead of keeping that callback
+  loop inline in `NodeCanvas::draw()`; broader canvas draw orchestration and
+  retained GL resource ownership remain.
 - 2026-06-16: `DEBT-0003` was narrowed again. Prepared-file background work
   now runs through an `AppRuntime`-owned queue/worker in
   `src/app_runtime.h/.cpp`, and `src/app_events.cpp` no longer defines a

docs/modernization/roadmap.md

@@ -117,8 +117,8 @@ Current architecture mismatches that must be treated as real blockers:
   checkerboard background setup now route through retained draw-merge helpers,
   with the cache-to-screen checkerboard-plane callback setup also reduced and
   the merged-path per-plane merged-texture draw plus the smoothing-mask face
-  shader/draw pass plus heightmap and current-mode callback setup now routed
-  through the same retained helper family.
+  shader/draw pass plus heightmap, current-mode, and grid-mode callback setup
+  now routed through the same retained helper family.
 - `app_layout.cpp` and `app_dialogs.cpp` are still mixed shell/controller files
   rather than thin composition/binding surfaces.
 - `App`, `Canvas`, `Node`, retained workers, and platform entrypoints still use
@@ -135,10 +135,11 @@ Current architecture mismatches that must be treated as real blockers:
   while the main Win32 entrypoint now groups window/GL/task/VR state behind a
   retained local state object instead of separate process-wide globals, the
   canvas async worker now sits behind a named retained local worker-state
-  helper instead of a bare static accessor, the prepared-file worker now lives
-  under `AppRuntime` instead of a retained static app-events worker, and
-  `App::rec_loop()` has a smaller local iteration/encode shell even though the
-  retained recording loop still owns the worker-side readback flow.
+  helper instead of a bare static accessor, the prepared-file worker and the
+  canvas async import/export/save/open worker now live under `AppRuntime`
+  instead of retained static app-events/canvas workers, and `App::rec_loop()`
+  now has a smaller local wait/iteration/encode shell even though the retained
+  recording loop still owns the worker-side readback flow.
 - Modern C++23 usage exists in extracted components, especially `std::span`,
   explicit result/status objects, and a few concepts, but the live app still
   does not consistently express ownership, thread affinity, or renderer

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`