3. postgres_migrate — versioned schema migrations
PostgreSQL glue for the provider-neutral migrations engine (daslib/sql_migrate
in the daslang repo, re-exported here): [sql_migration],
migrate_to_latest / try_migrate_to_latest, baseline,
[struct_convert] and convert_and_rename all work through this module.
Concurrent migrators coordinate on a session advisory lock keyed on the current
database: the strict forms block until the lock frees (session locks
auto-release when their holder crashes or disconnects), while the try_
siblings fail fast.