Extract stroke preview final composite helper

docs/modernization/roadmap.md

@@ -3097,6 +3097,10 @@ Results:
   shares the retained stroke execution helper callback surface, while shader
   setup, pad color selection, framebuffer ownership, and final OpenGL draw
   remain in the legacy Canvas path.
+- `NodeStrokePreview` final composite sampler/input binding and texture-slot
+  intent now share one local retained helper, while mixer execution,
+  per-sample stroke callbacks, framebuffer copies, and final OpenGL draw
+  ownership remain in the preview node.
 - `Canvas::stroke_draw` pad-region planning now shares the retained stroke
   execution helper wrapping `pp_paint_renderer`, while pad color selection,
   dirty-face iteration, framebuffer copies, quad upload, and draw execution