Another engine's datadir mounts cleanly and reads as uninitialised to
rds-init, which would initialise beside the customer's data and serve an
empty database that passes every health probe while the real data sits
unreferenced.
rds-init now writes the engine to ${DATA_MOUNT}/.spinifex-rds/engine
before the datadir is inspected, backfills it on attach for volumes
predating it, and refuses a stamp that disagrees.
DBName was passed through unvalidated, so a name such as my-db was accepted
at CreateDBInstance and only quoted into safety by psql inside the guest.
Engine gains a validateDBName rule and ValidateDBName, and rds-init re-checks
the same rule in load_handoff before initdb, where a failure would clear the
datadir and spend the one-shot master password.
PostgreSQL caps the name at NAMEDATALEN-1.
Julian Sommer