Extract Canvas draw_merge layer texture helper

docs/modernization/roadmap.md

@@ -67,6 +67,9 @@ Recent 2026-06-13 retained preview reductions continue to narrow DEBT-0036:
 feedback/material/composite planning and stroke-shader uniform assembly through
 `plan_legacy_node_stroke_preview_pass_orchestration(...)`, leaving the preview
 node with a smaller live-GL callback surface around pass execution.
+`Canvas::draw_merge()` also now routes its standard per-layer texture-alpha pass
+through `execute_legacy_canvas_draw_merge_layer_texture(...)`, reducing another
+piece of retained layer-composite sequencing.
 
 ## Target Component Architecture