Transactional layer over the storage tier: a multi-version path for OLTP traffic that needs snapshot
isolation and a single-version path for workloads where versioning is pure overhead, both behind one
Transaction handle.
Invariant: a TransactionId is system-wide unique and monotonic via Uuid7. CDC, replication and
subscriptions order events by it, so reusing or back-dating an id silently breaks consumer consistency.