Extract Canvas draw_merge layer texture helper

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` now routes the standard per-layer
+  texture-alpha pass through
+  `execute_legacy_canvas_draw_merge_layer_texture(...)`; temporary-stroke
+  branches, per-plane iteration, and concrete RTT ownership remain local to
+  `Canvas`. Next slice should target another narrow draw-merge execution seam
+  without reopening landed temporary-composite, layer-blend, final-plane, or
+  texture-alpha helpers.
 - 2026-06-13: `NodeStrokePreview::draw_stroke_immediate()` now routes retained
   preview feedback/material/composite planning plus stroke-shader uniform
   assembly through `plan_legacy_node_stroke_preview_pass_orchestration(...)`;