Thin canvas draw seams and own grid worker

docs/modernization/roadmap.md

@@ -102,13 +102,16 @@ Current architecture mismatches that must be treated as real blockers:
   app shell.
 - `pp_panopainter_ui` still depends on `pp_legacy_app`.
 - `Canvas`, `NodeCanvas`, and `NodeStrokePreview` still own too much live
-  OpenGL execution around the renderer boundary.
+  OpenGL execution around the renderer boundary, even though `NodeCanvas`
+  display resolve, cache-to-screen composite, and post-draw mask/grid/current-
+  mode sequencing now route through retained draw-merge helpers.
 - `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, retained static worker
   ownership in several app families, and ad hoc mutex/condition-variable
-  ownership.
+  ownership, even though most previously detached or raw app-facing worker
+  launches now use owned `std::jthread` or service-owned worker queues.
 - 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