Share retained stroke preview live pass helper

docs/modernization/roadmap.md

@@ -3103,6 +3103,10 @@ Results:
   assembly now shares one retained utility, while shader activation timing,
   texture/sampler binding, framebuffer ownership, pad execution, and final
   OpenGL draw ordering remain in the legacy Canvas path.
+- `NodeStrokePreview` dual-pass and main-pass frame-loop execution plus
+  full-frame copy-back now share one local retained helper surface, while
+  checkerboard/background capture ordering, texture-unit binding, mixer
+  ownership, and final composite semantics 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