MigrationRunner applies pending migrations through the migration ledger
exactly once; when a migration fails it throws a MigrationFailedError
whose diagnostic is a structured object identifying the failing migration
(version), the failure phase (apply/record), the underlying cause, and the
applied/pending ledger state, serializable via toJSON. Re-exported from
the driver boundary so no other package needs the pg driver to run
migrations. Advisory lock (T04) and ready gate (T06) remain out of scope.
MigrationLedger over the package-owned pg Pool: ensure() creates the
schema_migrations table (version text PRIMARY KEY, applied_at timestamptz
NOT NULL DEFAULT now()) with idempotent DDL; record() inserts an applied
migration with a parameterized, idempotent statement (ON CONFLICT DO
NOTHING — a rerun never double-applies); has()/applied() read the ledger
back in apply order. Re-exported from the driver boundary (src/index.ts)
so no other package needs the pg driver to touch migration state.
implementer