Extract Canvas draw_merge final plane composite helper

docs/modernization/tasks.md

@@ -528,6 +528,13 @@ Progress Notes:
   merged-texture copy/grid/final-redraw seam or another similarly narrow final
   composite boundary without reopening landed temporary-composite or
   per-layer blend helpers.
+- 2026-06-13: `Canvas::draw_merge()` end-of-plane merged-texture copy,
+  optional checkerboard redraw, and final merged-texture composite ordering now
+  route through `execute_legacy_canvas_draw_merge_final_plane_composite(...)`;
+  per-plane iteration and concrete framebuffer, sampler, texture, and draw
+  callbacks remain local to `Canvas`. Next slice should target another narrow
+  retained draw-merge boundary without reopening landed temporary-composite,
+  layer-blend, or final-plane helpers.
 - 2026-06-13: `Canvas::draw_merge()` erase live temporary-stroke composite now
   routes retained setup, sampler bind, texture bind, draw, and texture unbind
   ordering through `execute_legacy_canvas_stroke_temporary_composite(...)`;