chat-store: flop envelopes in handle-messages so backlog comes in correct order
=/ mailbox=(unit mailbox) (~(get by inbox) path.act)
?~ mailbox
[~ state]
+ =. envelopes.act (flop envelopes.act)
=/ evaluated-envelopes=(list envelope) ~
|- ^- (quip card _state)
?~ envelopes.act
