The controller sends its KEM offer both on its own guard event and in reply
to the responder's offer. SignalOffer was idempotent only while awaiting the
answer; once the answer arrived (awaitingRekey) a repeat call started a fresh
exchange with a different PSK, desyncing the two peers (one on the old PSK,
one on the new) so WireGuard derived misaligned transport keys and dropped all
data. Treat awaitingRekey as in-flight too and return the same offer.