Extract draw merge temporary composite wrapper

docs/modernization/debt.md

@@ -76,6 +76,10 @@ agent or engineer to remove them without reconstructing context from chat.
   `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. `Canvas::draw_merge()` temporary
+  composite setup now routes through `execute_canvas_draw_merge_temporary_composite(...)`;
+  setup, sampler, texture, draw, and unbind callbacks still remain retained 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/tasks.md

@@ -655,6 +655,11 @@ Progress Notes:
   `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: `Canvas::draw_merge()` temporary composite setup now routes
+  through `execute_canvas_draw_merge_temporary_composite(...)`; the retained
+  path still owns the concrete setup, sampler, texture, draw, and unbind
+  callbacks. Next slice should target another narrow draw-merge seam without
+  reopening the landed temporary-composite helper.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained frame-plan assembly for previous-sample projection mode and zoom
   scaling. Next slice should target the remaining preview/Canvas stroke