Thin recording bridge and canvas draw seams

docs/modernization/tasks.md

@@ -151,6 +151,10 @@ Current slice:
   callback setup now also routes through
   `make_legacy_canvas_draw_merge_grid_modes_draw(...)`, but broader post-draw
   orchestration is still inline.
+- `NodeCanvas` checkerboard background plane callback setup now also routes
+  through `make_legacy_canvas_draw_merge_background_checkerboard_plane(...)`,
+  but the node still owns broader live layer traversal and renderer-state
+  sequencing.
 - `NodeCanvas` smoothing-mask face shader setup plus per-face draw execution
   now also route through
   `execute_legacy_canvas_draw_merge_smask_faces(...)`, but the node still owns
@@ -420,12 +424,10 @@ Current slice:
 - preview background rendering, recording, and the retained
   `NodePanelGrid::bake_uvs()` worker now also use `std::jthread`, but their
   retained loop/control flow is still open
-- `App::rec_loop()` now routes its frame encode/update chunk through a local
-  helper, its iteration-context setup now also routes through a local helper,
-  and its wait/plan/encode iteration shell now also routes through a local
-  helper, while `App::update()` no longer carries the dead update mutex
-  residue; retained recording loop control and readback ownership are still
-  open
+- `App::rec_loop()` now delegates its worker-iteration orchestration into the
+  retained recording bridge in `src/legacy_recording_services.cpp`, while
+  `App::update()` no longer carries the dead update mutex residue; retained
+  recording loop control, readback ownership, and MP4 execution are still open
 
 Write scope:
 - `src/canvas.cpp`