Own preview and recording workers

docs/modernization/tasks.md

@@ -49,7 +49,9 @@ Completed, blocked, and superseded task history moved to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 preview and recording code
+  - app-facing detached launches are no longer the main issue; preview and
+    recording now use owned worker threads, but those families still rely on
+    retained static/global ownership and ad hoc runtime control
   - 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
@@ -359,7 +361,8 @@ Current slice:
   workers with explicit UI-thread handoff
 - 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
+- preview background rendering and recording thread ownership now also use
+  `std::jthread`, but their retained loop/control flow is still open
 
 Write scope:
 - `src/canvas.cpp`