Snapshot of the mutable in-memory state taken at BEGIN time so
ROLLBACK can restore it. See begin_transaction, rollback_transaction.
tables is deep-cloned (the Table::deep_clone helper reallocates
the Arc<Mutex<_>> row storage so snapshot and live state don’t
share a map).
Default fraction of free pages that triggers an auto-VACUUM after
a page-releasing DDL (DROP TABLE / DROP INDEX / ALTER TABLE DROP
COLUMN). Matches SQLite’s classic 25% heuristic. Override per
connection with Database::set_auto_vacuum_threshold (or
Connection::set_auto_vacuum_threshold); pass None to disable.