Constant HIGHWATER_STATE_KEY 

pub const HIGHWATER_STATE_KEY: &str = "hlc_highwater";

protocol_state key under which the clock’s high-water mark is persisted, so it cannot regress across restarts (see Hlc::seed). Accepted row and Circle metadata floors commit with their data; cycle-end flushes also retain locally minted timestamps. Every write raises the persisted floor monotonically.