Extract Canvas draw_merge layer blend helper

docs/modernization/tasks.md

@@ -509,6 +509,15 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` per-layer blend composite now routes
+  merge-RTT unbind, shader setup, optional destination-copy feedback, draw,
+  and cleanup ordering through
+  `execute_legacy_canvas_draw_merge_layer_blend(...)`; temporary-stroke
+  branch selection, layer iteration, and final merged-plane redraw ownership
+  remain local to `Canvas`. Next slice should target the remaining end-of-plane
+  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()` 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(...)`;