Verify an echoed wire blob against the current subject/toolName and
return the inner plaintext state, or Nullable!string.init on any failure
(bad MAC, GCM auth failure, expired, wrong-subject bind, or unparseable).
Fail-closed: the caller re-elicits on null.
Verify an echoed wire blob against the current subject/toolName and return the inner plaintext state, or Nullable!string.init on any failure (bad MAC, GCM auth failure, expired, wrong-subject bind, or unparseable). Fail-closed: the caller re-elicits on null.