bumpRollbackCounter

Atomically increment and persist the rollback counter, returning the new value. Never decreases. Called on every send and every ratchet-advancing receive (one advance each), not on export; exportStateSealed binds the current value. Because it tracks ratchet advances, a restore of any state older than the latest send presents a lower counter and is caught. Committing here is a per-message secure-storage write, so keep it fast.