Promote draw merge final plane composite task

docs/modernization/tasks.md

@@ -2260,6 +2260,34 @@ Validation:
 ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-onfailure
 ```
 
+### STR-054 - Extract Draw Merge Final Plane Composite Execution
+
+Status: Ready
+Score: no score movement
+Debt: `DEBT-0036`
+Scope: `src/canvas.cpp`, `src/legacy_canvas_draw_merge_services.h`, `tests/paint_renderer/compositor_tests.cpp`
+
+Goal:
+
+Move the remaining `Canvas::draw_merge_final_plane_composite()` execution
+bundle into retained helpers so the final composite wrapper keeps only request
+assembly and dispatch.
+
+Done Checks:
+
+- `Canvas::draw_merge_final_plane_composite()` no longer owns the final-plane
+  execution bundle inline.
+- Regression coverage proves the extracted helper preserves final composite
+  sampler, texture, draw, and unbind ordering.
+- `docs/modernization/debt.md` records the reduced draw-merge final-plane
+  surface.
+
+Validation:
+
+```powershell
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-onfailure
+```
+
 ### STR-010 - Extract Remaining Draw Merge Composite Orchestration
 
 ### STR-016 - Extract Draw Merge Layer Composite Execution