Extract draw merge temporary branch helper

docs/modernization/debt.md

@@ -80,6 +80,11 @@ agent or engineer to remove them without reconstructing context from chat.
   composite setup now routes through `execute_canvas_draw_merge_temporary_composite(...)`;
   setup, sampler, texture, draw, and unbind callbacks still remain retained in
   the legacy Canvas path.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
+  erase and paint branch wiring now routes through
+  `execute_legacy_canvas_draw_merge_temporary_composite(...)`; the retained
+  path still owns the concrete setup, sampler, texture, draw, and unbind
+  callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes retained preview feedback/material/composite planning plus stroke
   shader uniform assembly through