Extract Canvas draw_merge plane setup helper

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` now routes per-plane merge-target clear,
+  blend-state gating, and optional checkerboard prepass through
+  `execute_legacy_canvas_draw_merge_plane_setup(...)`; per-layer iteration,
+  temporary-stroke branches, and concrete framebuffer ownership remain local to
+  `Canvas`. Next slice should target another narrow draw-merge execution seam
+  without reopening landed plane-setup, temporary-composite, layer-blend,
+  final-plane, or texture-alpha helpers.
 - 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