use AshReplicant.Checkpoint, repo: MyApp.Repo, domain: MyApp.Domain generates the
bundled AshPostgres checkpoint resource (table ash_replicant_checkpoints) bound to
the host's repo and domain.
One row per replication slot: slot_name (primary key) and the durable commit_lsn
watermark. The sink upserts it in the same transaction as the mirrored changes, which
is what gives effect-once (dup = 0) semantics.
A macro is required because an AshPostgres resource needs its host repo at compile
time; ash_replicant cannot hardcode it.