Promote draw merge paint branch task

docs/modernization/tasks.md

@@ -1402,6 +1402,37 @@ ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_composito
 | --- | --- | ---: | --- | --- |
 | 2026-06-13 | STR-017 | +1 renderer boundary and OpenGL parity | `ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure` | `e8fdd96d` |
 
+### STR-018 - Extract Draw Merge Temporary Paint Composite Branch
+
+Status: Done
+Score: +1 renderer boundary and OpenGL parity
+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 paint branch inside `Canvas::draw_merge()` into a
+retained helper so the callsite keeps only branch selection and concrete GL
+object wiring.
+
+Done Checks:
+
+- `Canvas::draw_merge()` no longer builds the temporary paint composite inline.
+- Regression coverage proves the extracted helper preserves paint-branch order.
+- `docs/modernization/debt.md` records the reduced draw-merge paint surface.
+
+Validation:
+
+```powershell
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
+```
+
+### Completed Task Log
+
+| Date | Task | Score | Validation | Commit |
+| --- | --- | ---: | --- | --- |
+| 2026-06-13 | STR-018 | +1 renderer boundary and OpenGL parity | `ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure` | `037be1a7` |
+
 ### STR-012 - Extract Preview Final Composite Orchestration
 
 Status: Done