Salvages #67559 — original gated sync_turn/on_memory_write/on_session_end but missed shutdown(), whose flush_all() still persisted on exit. hermes-sweeper review (salvageability=high) flagged this as the one gap.
Guard sits after the worker-thread joins, not at the top: cleanup is independent of persistence, and a top-of-method return would leak _prefetch_thread/_sync_thread. Adds TestShutdown and clarifies the saveMessages=false README row.
Credit @Matroskin86 (original PR author).
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