The LocalityProviderExecutor provides locality for operators during backfilling.
It buffers input data into a state table using locality columns as primary key prefix.
Apply one buffered change to the per-key compaction slot, mirroring the semantics of
ChangeBuffer: consecutive changes of the same key are merged into at most one record.
Op code of a buffered change, stored in the _rw_op column of the sort buffer table.
Update ops are normalized to Insert/Delete on write (see Op::normalize_update),
and reconstructed as Update records by per-key compaction on replay.