Route Canvas erase setup through helper

docs/modernization/roadmap.md

@@ -3009,6 +3009,10 @@ Results:
   shader setup helper for `CompErase` uniforms, while keeping per-onion-frame
   alpha updates, texture binding, temporary erase texture selection, and draw
   execution in the retained node path.
+- `Canvas::stroke_commit` and `Canvas::draw_merge` retained erase compositing
+  now share the stroke erase shader setup helper, while texture binding, mask
+  RTT binding, dirty/layer mutation, framebuffer feedback, and draw execution
+  remain in retained Canvas code.
 - `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