Promote draw merge plane iteration task

docs/modernization/tasks.md

@@ -2021,6 +2021,34 @@ ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_composito
 cmake --build --preset windows-msvc-default --config Debug --target PanoPainter
 ```
 
+### STR-046 - Extract Draw Merge Plane Iteration Orchestration
+
+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()` plane iteration and branch
+orchestration into a retained helper so the callsite keeps only framebuffer
+setup, per-plane dispatch, and final composite gating.
+
+Done Checks:
+
+- `Canvas::draw_merge()` no longer owns the per-plane iteration and branch
+  orchestration inline.
+- Regression coverage proves the helper preserves plane dispatch order and
+  final composite gating.
+- `docs/modernization/debt.md` records the reduced draw-merge plane-loop
+  surface.
+
+Validation:
+
+```powershell
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
+```
+
 ### STR-010 - Extract Remaining Draw Merge Composite Orchestration
 
 ### STR-016 - Extract Draw Merge Layer Composite Execution