Extract Canvas draw_merge final plane composite helper

docs/modernization/roadmap.md

@@ -3150,6 +3150,11 @@ Results:
   ordering through `execute_legacy_canvas_draw_merge_layer_blend(...)`, while
   temporary-stroke branch selection, framebuffer copies, and final merged-plane
   ownership remain in the legacy Canvas path.
+- `Canvas::draw_merge()` end-of-plane merged-texture copy plus optional
+  checkerboard/final-texture redraw ordering now routes through
+  `execute_legacy_canvas_draw_merge_final_plane_composite(...)`, while
+  per-plane iteration and the concrete framebuffer, sampler, texture, and draw
+  callbacks remain in the legacy Canvas path.
 - `NodeStrokePreview::draw_stroke_immediate()` now shares
   `execute_legacy_node_stroke_preview_pass_sequence(...)` for
   dual-pass/background/main-pass/final-composite/copy-back ordering, while the