markRemoteIdentityVerified

Records that the user compared safetyNumber with the other device and confirmed it: the pinned identity of the remote device in it becomes VerificationState.VERIFIED. Bound to that exact key: if the pin changed since safetyNumber was derived, this throws SecureMessageClientException.RemoteIdentityConflict and changes nothing. A later accepted identity change resets it to VerificationState.UNVERIFIED.