Route layer merge composite setup through helper

docs/modernization/roadmap.md

@@ -2984,6 +2984,10 @@ Results:
   through retained callbacks, so the legacy body no longer owns the loop order
   directly. The callbacks still execute the existing OpenGL RTT, texture,
   sampler, shader, history, and layer mutation work under DEBT-0036.
+- `Canvas::layer_merge` now shares the retained stroke composite shader helper
+  for its source-over-destination `CompDraw` uniform setup, while keeping the
+  existing layer dirty mutation, framebuffer copy, texture binding, and draw
+  order local to the Canvas layer-merge path.
 - 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.