- Introduce MongoClientManager: owns a shared AsyncMongoClient per process,
  with idempotent start/close under asyncio.Lock, ping/pool_stats, and
  crud / sync_crud / long_running_crud factories that inject the client
  into lightweight CRUD instances.
- BaseCrud accepts client= (keyword-only); _owns_client flag drives whether
  __aenter__/__aexit__ create and close the connection. Legacy
  connection_string mode unchanged.
- Every public CRUD method gains an optional database= kwarg; resolution
  goes through _resolve_db(database) with fallback to self.db /
  default_database and a clear LibMongoError otherwise. GridFS cache keyed
  by (collection, suffix, database).
- switch_database now raises in shared-client mode to prevent races on the
  shared self.db; legacy path retained.
- Expose MongoClientManager from package root; bump version to 1.1.0.
- New test_singleton.py covers injection, _resolve_db, database= per-call,
  manager lifecycle/race lock, ping, factories, and long-running client.
- README + CHANGELOG document the shared-client pattern and migration.