index

BIP-44 leaf index of the vault this contract currently points at. Starts at 0. Moved by nextDestination (advance by one), the buildDestination index overload (jump to a chosen leaf), and load (restore from a saved record). Persisted by save.

Backed by an atomic so concurrent nextDestination calls (or other cross-thread mutations) produce distinct values instead of racing on a plain read-modify-write.