verifySignature accepts a signature whose key has expired, since a
manifest outlives its key. The OpenPGP library stops at the expired key
without checking the signature's own expiry time, so a signature made
with one was refused while the key was live and accepted once the key
had expired as well. verifySignature now gets the signature packet from
openpgp.VerifyDetachedSignature and refuses it when it has expired.
docs/FORMAT.md Signature Scheme states the rule.
Model: opus-5-5
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