Move canvas async work into app runtime

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