Extract Canvas draw_merge layer blend helper

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