Own preview and recording workers

docs/modernization/roadmap.md

@@ -106,9 +106,9 @@ Current architecture mismatches that must be treated as real blockers:
 - `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
-  global singleton reach, raw observer pointers, detached `std::thread`
-  launches in preview/recording paths, and ad hoc
-  mutex/condition-variable ownership.
+  global singleton reach, raw observer pointers, retained static worker
+  ownership in several app families, and ad hoc mutex/condition-variable
+  ownership.
 - 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

@@ -49,7 +49,9 @@ Completed, blocked, and superseded task history moved to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`