A Lance write that has produced fragment files on object storage but is
not yet committed to the dataset’s manifest. The staged-write primitives
are consumed by MutationStaging (exec/staging.rs,
exec/mutation.rs) and the bulk loader (loader/mod.rs). The
intent: defer Lance commits to end-of-query so a mid-query failure
leaves the touched table at the pre-mutation HEAD instead of
drifting ahead. See docs/runs.md for the publisher-CAS contract
this builds on.