Extract Canvas draw_merge layer texture 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
+  the standard per-layer texture-alpha pass through
+  `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,
+  temporary-stroke branch selection, and concrete layer RTT ownership remain in
+  the legacy Canvas path.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes retained preview feedback/material/composite planning plus stroke
   shader uniform assembly through

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
 

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(...)`;