screen: signatures required as the policy a change came under says

Review 3, R1: what a node received was vouched for only while its own
document's policy required signatures. A joiner's first sync, and
every node's message bringing `required`, went in by the rules alone:
a malicious first peer could plant changes under other members' actors
at their next seqs, their real ones then failing as duplicates for
good.

The signatures a message carries are always offered now, and the
screen always has them: a change waits for one where the policy at the
heads it is judged at requires it (rules::Reads::required, read once
per heads), whatever the receiving document holds. Changes written
under a policy not requiring signatures wait for none, as on every
other node; the check for forged changes runs always. Changes waiting
for a signature are judged again whenever signatures come.

The test of signatures required now takes the history into an empty
document; a new one takes changes written under a policy not requiring
them without any.

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