Promote draw merge layer composite task

docs/modernization/tasks.md

@@ -1325,6 +1325,39 @@ Completed Task Log:
 
 ### STR-010 - Extract Remaining Draw Merge Composite Orchestration
 
+### STR-016 - Extract Draw Merge Layer Composite Execution
+
+Status: Ready
+Score: +2 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 large per-layer `draw_merge()` composite execution block into a
+retained helper so the callsite only supplies branch selection and concrete GL
+objects.
+
+Done Checks:
+
+- `Canvas::draw_merge()` no longer builds the per-layer composite execution
+  inline.
+- Regression coverage proves the extracted helper preserves per-branch order.
+- `docs/modernization/debt.md` records the reduced draw-merge composite
+  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-016 | +2 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` | `pending` |
+
 ### STR-012 - Extract Preview Final Composite Orchestration
 
 Status: Done