Extract draw merge layer composite orchestration

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
+  the remaining per-layer branch orchestration through
+  `execute_legacy_canvas_draw_merge_layer_composite(...)`; the retained path
+  still owns the concrete temporary-stroke, layer-texture, and layer-blend
+  callback bodies.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_commit()` now
   builds its retained callback table through
   `make_legacy_canvas_stroke_commit_callbacks(...)`; the legacy executor still