Superseding a stale micro marker joins the now-adjacent user turns into one
model-facing row, while the originals stay in display history as compacted
rows, so resumed display history painted every merged input twice (and one
more time per later pass). Flag the join display_metadata.model_only and skip
it in every display projection: resume dedupe, the indexed and legacy
get_messages pages, and the prompt timeline. The model payload is unchanged.
