Route stroke preview checkerboard setup through helper

docs/modernization/roadmap.md

@@ -2997,6 +2997,10 @@ Results:
   optional `TexBG` uniform when copy-based blend destination feedback is active.
   Layer traversal, framebuffer copies, sampler/texture binding, and draw
   ordering remain in the legacy Canvas path.
+- `NodeStrokePreview` preview background drawing now shares the retained
+  draw-merge shader setup helper for checkerboard uniforms, preserving its
+  colorize condition and preview MVP while leaving capture ordering, texture
+  copies, stroke setup, and draw execution in the retained preview path.
 - `Canvas::stroke_draw_mix` now shares the retained stroke composite shader
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture