Extract Canvas draw_merge final plane composite helper

docs/modernization/debt.md

@@ -301,6 +301,12 @@ agent or engineer to remove them without reconstructing context from chat.
   through `legacy_canvas_draw_merge_services.h`. The retained Canvas path still
   owns draw-merge layer iteration, blend-gate branching, framebuffer copies,
   sampler/texture binding, and draw ordering.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` end-of-plane
+  merged-texture copy, optional checkerboard redraw, and final merged-texture
+  composite ordering now route through
+  `execute_legacy_canvas_draw_merge_final_plane_composite(...)`; the retained
+  Canvas path still owns per-plane iteration plus the concrete framebuffer,
+  sampler, texture, and draw callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::layer_merge` now reuses
   `legacy_canvas_stroke_composite_services.h` for retained layer-merge
   `kShader::CompDraw` binding and source/destination blend uniform writes.

docs/modernization/roadmap.md

@@ -3150,6 +3150,11 @@ Results:
   ordering through `execute_legacy_canvas_draw_merge_layer_blend(...)`, while
   temporary-stroke branch selection, framebuffer copies, and final merged-plane
   ownership remain in the legacy Canvas path.
+- `Canvas::draw_merge()` end-of-plane merged-texture copy plus optional
+  checkerboard/final-texture redraw ordering now routes through
+  `execute_legacy_canvas_draw_merge_final_plane_composite(...)`, while
+  per-plane iteration and the concrete framebuffer, sampler, texture, and draw
+  callbacks remain in the legacy Canvas path.
 - `NodeStrokePreview::draw_stroke_immediate()` now shares
   `execute_legacy_node_stroke_preview_pass_sequence(...)` for
   dual-pass/background/main-pass/final-composite/copy-back ordering, while the

docs/modernization/tasks.md

@@ -528,6 +528,13 @@ Progress Notes:
   merged-texture copy/grid/final-redraw seam or another similarly narrow final
   composite boundary without reopening landed temporary-composite or
   per-layer blend helpers.
+- 2026-06-13: `Canvas::draw_merge()` end-of-plane merged-texture copy,
+  optional checkerboard redraw, and final merged-texture composite ordering now
+  route through `execute_legacy_canvas_draw_merge_final_plane_composite(...)`;
+  per-plane iteration and concrete framebuffer, sampler, texture, and draw
+  callbacks remain local to `Canvas`. Next slice should target another narrow
+  retained draw-merge boundary without reopening landed temporary-composite,
+  layer-blend, or final-plane helpers.
 - 2026-06-13: `Canvas::draw_merge()` erase live temporary-stroke composite now
   routes retained setup, sampler bind, texture bind, draw, and texture unbind
   ordering through `execute_legacy_canvas_stroke_temporary_composite(...)`;