Promote draw merge per-plane setup task

docs/modernization/tasks.md

@@ -2230,6 +2230,34 @@ Validation:
 ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-onfailure
 ```
 
+### STR-053 - Extract Draw Merge Per-Plane Setup And Unbind
+
+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 per-plane framebuffer setup and unbind handling used by
+`execute_canvas_draw_merge_plane_dispatch()` into retained helpers so the
+per-plane wrapper keeps only branch dispatch and final composite gating.
+
+Done Checks:
+
+- `execute_canvas_draw_merge_plane_dispatch()` no longer owns the per-plane
+  framebuffer setup and unbind handling inline.
+- Regression coverage proves the extracted helper preserves plane framebuffer
+  setup, branch dispatch, and final composite gating.
+- `docs/modernization/debt.md` records the reduced draw-merge plane setup
+  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