Add stroke mix order regression

docs/modernization/debt.md

@@ -525,6 +525,10 @@ agent or engineer to remove them without reconstructing context from chat.
   routes retained mix-shell execution through a local wrapper around
   `execute_legacy_canvas_stroke_mix_pass_shell(...)`; the live path still owns
   the concrete mixer framebuffer setup and GL capability toggles.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_mix()` now
+  has regression coverage for retained mixer-state callback ordering through
+  `execute_legacy_canvas_stroke_mix_pass(...)`; the live path still owns the
+  concrete mixer framebuffer setup and GL capability toggles.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` checkerboard
   background shader setup and final merged-texture redraw setup now route
   through `legacy_canvas_draw_merge_services.h`. The retained Canvas path still

docs/modernization/tasks.md

@@ -842,6 +842,39 @@ Progress Notes:
   slice should target the remaining `stroke_draw_samples()` callback body or
   any final temporary-texture composite setup without reopening landed
   sample, sampler, dirty, face, or pad helpers.
+
+### STR-019 - Extract Stroke Draw Mix Mixer State And Copy Ordering
+
+Status: Done
+Score: +1 renderer boundary and OpenGL parity
+Debt: `DEBT-0036`
+Scope: `src/canvas.cpp`, `src/legacy_canvas_stroke_execution_services.h`, `tests/paint_renderer/compositor_tests.cpp`
+
+Goal:
+
+Move the remaining `stroke_draw_mix()` mixer-framebuffer state and copy-order
+callbacks into a retained helper so `Canvas::stroke_draw_mix()` keeps only
+branch selection plus concrete GL object wiring.
+
+Done Checks:
+
+- `Canvas::stroke_draw_mix()` no longer owns the remaining mixer framebuffer
+  state and copy ordering inline.
+- Regression coverage proves the extracted helper preserves the mixer-state
+  callback order.
+- `docs/modernization/debt.md` records the reduced stroke-mix shell 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-019 | +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` |
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained preview background capture ordering, final composite ordering, and
   preview texture-copy bind-before-copy behavior via