Share retained stroke mix pass helper

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::stroke_draw_mix()` now routes visible-plane filtering,
+  retained sampler/texture-slot binding, and final plane draw ordering through
+  `execute_legacy_canvas_stroke_mix_pass(...)`; mixer framebuffer/state setup
+  and per-plane shader material/MVP preparation remain local to `Canvas`. Next
+  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.
 - 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