Share retained stroke mix pass helper

docs/modernization/roadmap.md

@@ -3128,6 +3128,11 @@ Results:
   triangulation, sample-point assembly, and retained destination-copy / upload
   / draw helper handoff, while direct GL callback wiring and the remaining
   live draw ownership stay in the legacy Canvas path.
+- `Canvas::stroke_draw_mix()` now shares
+  `execute_legacy_canvas_stroke_mix_pass(...)` for visible-plane filtering,
+  retained sampler/texture-slot binding, and final plane draw ordering, while
+  mixer framebuffer/state setup and per-plane shader material/MVP preparation
+  remain in the legacy Canvas path.
 - `NodeStrokePreview::stroke_draw_mix()` now shares one local helper for mixer
   framebuffer bind/unbind, viewport/scissor/blend state, texture-slot
   binding, and final plane draw, while material planning and shader uniform