Extract Canvas draw_merge layer blend helper

docs/modernization/debt.md

@@ -18,6 +18,13 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `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(...)`; layer iteration,
+  temporary-stroke branch selection, framebuffer copies, sampler/texture
+  object callbacks, and final merged-plane ownership remain retained in
+  `Canvas`.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` erase live
   temporary-stroke composite now routes retained setup, sampler bind, texture
   bind, draw, and texture unbind ordering through