Confirms Lance v4.0.0 has row-level CAS for merge_insert only when the
join-key column carries lance-schema:unenforced-primary-key=true. Our
__manifest schema does not, so the publisher silently allows duplicate
object_id rows under concurrent writers. Note + reproducible scratch
crate select the layered (pre-check + row-level CAS) approach for the
publisher API ticket.
