Extract preview mix pass execution

docs/modernization/tasks.md

@@ -1870,7 +1870,7 @@ ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_stroke_ex
 
 ### STR-041 - Extract Preview Mix Pass Execution Ownership
 
-Status: Ready
+Status: Done
 Score: no score movement
 Debt: `DEBT-0036`
 Scope: `src/node_stroke_preview.cpp`, `tests/paint_renderer/compositor_tests.cpp`
@@ -1881,6 +1881,8 @@ Move the remaining `NodeStrokePreview::stroke_draw_mix()` execution ownership
 into a retained helper so the callsite keeps only the structural mix-pass
 orchestration.
 
+Closeout: `cf859cd4`
+
 Done Checks:
 
 - `NodeStrokePreview::stroke_draw_mix()` no longer owns the mix-pass execution