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

docs/modernization/roadmap.md

@@ -3145,6 +3145,11 @@ Results:
   `execute_legacy_canvas_stroke_temporary_composite(...)`, leaving only the
   concrete GL object callbacks and broader final composite ownership in the
   legacy Canvas path.
+- `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(...)`, while
+  temporary-stroke branch selection, framebuffer copies, and final merged-plane
+  ownership remain in the legacy Canvas path.
 - `NodeStrokePreview::draw_stroke_immediate()` now shares
   `execute_legacy_node_stroke_preview_pass_sequence(...)` for
   dual-pass/background/main-pass/final-composite/copy-back ordering, while the

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(...)`;