Promote draw merge per-plane dispatch task

docs/modernization/tasks.md

@@ -2200,6 +2200,34 @@ Validation:
 ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-onfailure
 ```
 
+### STR-052 - Extract Draw Merge Per-Plane Dispatch Wrapper
+
+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 dispatch wrapper used by
+`execute_canvas_draw_merge_plane_iteration()` into retained helpers so the
+plane loop keeps only plane selection and helper dispatch.
+
+Done Checks:
+
+- `execute_canvas_draw_merge_plane_iteration()` no longer owns the per-plane
+  dispatch wrapper inline.
+- Regression coverage proves the extracted helper preserves per-plane
+  framebuffer setup, branch dispatch, and final composite gating.
+- `docs/modernization/debt.md` records the reduced draw-merge plane-dispatch
+  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