close

Release resources. Suspends because closing the dust state must be serialized against an in-flight balance: balanceAgainst reads the shared DustLocalState's nullifiers under balanceMutex, so closing it out from under a running balance throws "DustLocalState has been closed" (regression — every other dust-close path already holds this mutex; the teardown one added later did not). Under localnet the balance is instant so the window never opened; on PreProd it spans seconds of dust sync, so a teardown mid-deploy raced the close. Acquiring the mutex here makes the teardown WAIT for the balance instead of yanking its state.