Share retained stroke preview sample helper

docs/modernization/roadmap.md

@@ -3115,6 +3115,10 @@ Results:
   binding/unbinding intent now shares retained stroke execution helpers, while
   sampler binding, concrete GL object mapping, framebuffer ownership, and final
   draw execution remain in the legacy Canvas path.
+- `NodeStrokePreview::stroke_draw_samples` now shares one local helper for
+  destination bind/unbind, framebuffer copy callback wrapping, sample-point
+  assembly, and brush-vertex upload/draw, while mixer-pass state execution and
+  higher-level pass orchestration remain in the preview node.
 - `Canvas::stroke_draw` pad-pass destination bind/copy/unbind ordering now
   shares the retained stroke execution helper callback surface, while shader
   setup, pad color selection, framebuffer ownership, and final OpenGL draw