screen: refuse a change at a seq this node made under its own actor

A node receiving a change of the actor it writes as, one it doesn't
hold, writes on as a fresh actor (another node writes as it, from a copy
of its files). Where the other node made it at a seq this node made a
change at too, the change was taken to Automerge, which refuses the
whole batch (duplicate seq): the message failed, and with it everything
else it carried. Signatures::forks found it forged only where a
signature verified a change of the actor at that seq, and this node
signs its own latest change only as it sends it.

forks now also finds a change forged where its actor is the one the
document writes as and the document's latest change of it
(get_last_local_change) is at its seq or later. apply_bytes switches to
the fresh actor after judging the batch, not before, so the check still
sees the actor; also where judging fails.

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