Share retained stroke sample and mix helpers

docs/modernization/roadmap.md

@@ -3123,6 +3123,15 @@ Results:
   texture-input dispatch now shares retained stroke execution helpers, while
   concrete GL object mapping, framebuffer ownership, shader timing, and final
   draw execution remain in the legacy Canvas path.
+- `Canvas::stroke_draw_samples()` now shares
+  `execute_legacy_canvas_stroke_sample_polygon(...)` for polygon
+  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.
+- `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
+  setup remain in the preview node.
 - `pp_paint_renderer_stroke_execution_tests` now covers retained stroke texture
   input binding order, sample execution destination-copy behavior, live-pass
   face-framebuffer dirty tracking, and pad-face destination-copy behavior