Promote draw merge temporary composite tasks

docs/modernization/tasks.md

@@ -2138,6 +2138,34 @@ Validation:
 ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
 ```
 
+### STR-050 - Extract Draw Merge Temporary Composite Dispatch Helpers
+
+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 temporary composite dispatch bodies used by
+`Canvas::draw_merge_branch_orchestration()` into retained helpers so the
+branch body keeps only branch selection and helper dispatch.
+
+Done Checks:
+
+- `execute_canvas_draw_merge_branch_body(...)` no longer owns the temporary
+  erase, temporary paint, texture, and blend dispatch bodies inline.
+- Regression coverage proves the extracted helpers preserve temporary erase,
+  temporary paint, texture, and blend execution order.
+- `docs/modernization/debt.md` records the reduced draw-merge temporary
+  composite 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