Own canvas async work and thin NodeCanvas composite

docs/modernization/tasks.md

@@ -49,8 +49,10 @@ Completed, blocked, and superseded task history moved to
   - `platform_legacy` is still part of the live app shell
 - The app runtime boundary is not finished:
   - render/UI queues are static `App` state
-  - detached workers still launch from canvas, preview, document export, and
-    recording code
+  - detached workers still launch from preview and recording code
+  - canvas async import/export/save/open now run through an owned in-file
+    worker, but their retained progress execution is still not a clean runtime
+    service boundary
   - thread-affinity rules are enforced by convention and asserts instead of
     explicit runtime contracts
 - The UI ownership boundary is not finished:
@@ -127,9 +129,9 @@ Current slice:
 - `NodeStrokePreview` final composite plus preview-texture copy now route
   through `legacy_node_stroke_preview_execution_services.h`, but the preview
   node still owns most live-pass and retained GL resource execution.
-- `NodeCanvas` display resolve for the `m_density != 1.f` path now routes
-  through `legacy_canvas_draw_merge_services.h`, but the cache-to-screen
-  composite block and broader canvas draw orchestration are still inline.
+- `NodeCanvas` display resolve plus cache-to-screen checkerboard/cache-texture
+  composite now route through `legacy_canvas_draw_merge_services.h`, but
+  broader canvas draw orchestration is still inline.
 
 Write scope:
 - `src/node_stroke_preview.cpp`
@@ -355,8 +357,9 @@ Current slice:
   UI-thread completion handoff
 - prepared-file save work and grid lightmap launch now also use service-owned
   workers with explicit UI-thread handoff
-- canvas, preview, document export, and recording-side detached work are still
-  open
+- canvas async import/export/save/open and timelapse export now also use owned
+  worker queues instead of detached threads
+- preview and recording-side detached work are still open
 
 Write scope:
 - `src/canvas.cpp`