Route stroke mixer composite setup through helper

docs/modernization/roadmap.md

@@ -2992,6 +2992,10 @@ Results:
   checkerboard backgrounds and the final merged texture redraw over the grid;
   layer traversal, blend destination copies, sampler/texture binding, and draw
   ordering remain in the legacy Canvas 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
+  binding, and draw execution remain retained.
 - Canvas thumbnail layer blending now uses the same canvas destination-feedback
   plan for framebuffer-fetch versus texture-copy decisions; the thumbnail draw
   itself still executes through retained OpenGL canvas code under DEBT-0036.