Extract draw merge erase branch helper

docs/modernization/debt.md

@@ -193,6 +193,11 @@ agent or engineer to remove them without reconstructing context from chat.
   the remaining temporary erase and paint callback bundle through
   `execute_legacy_canvas_draw_merge_temporary_composite(...)`; branch
   selection remains in `Canvas`.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
+  erase branch execution now routes through
+  `make_legacy_canvas_draw_merge_temporary_erase_composite(...)`; the live
+  path still owns the concrete layer RTT, mixer RTT, sampler, and plane
+  callbacks.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
   the layer-composite shell through a local wrapper around
   `execute_legacy_canvas_draw_merge_layer_composite(...)`; the final branch

docs/modernization/tasks.md

@@ -1327,10 +1327,11 @@ Completed Task Log:
 
 ### STR-016 - Extract Draw Merge Layer Composite Execution
 
-Status: Ready
+Status: Blocked
 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`
+Blocked By: Helper API shape mismatch in the current per-layer composite extraction attempt
 
 Goal:
 
@@ -1366,6 +1367,37 @@ ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_composito
 | --- | --- | ---: | --- | --- |
 | 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-017 - Extract Draw Merge Temporary Erase 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 erase 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 erase composite inline.
+- Regression coverage proves the extracted helper preserves erase-branch order.
+- `docs/modernization/debt.md` records the reduced draw-merge erase 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-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` | `pending` |
+
 ### STR-012 - Extract Preview Final Composite Orchestration
 
 Status: Done