Extract Canvas draw_merge plane setup helper

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `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-plane iteration, per-layer branch selection, and concrete framebuffer
+  ownership remain in the legacy Canvas path.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
   the standard per-layer texture-alpha pass through
   `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,

docs/modernization/roadmap.md

@@ -67,6 +67,10 @@ 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()` now also routes its per-plane merge-target clear,
+blend-state gating, and optional checkerboard prepass through
+`execute_legacy_canvas_draw_merge_plane_setup(...)`, reducing another retained
+plane-setup branch.
 `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.

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