Refuse a signature made when its key was not valid (closes #209)

verifySignature lets a key that has expired since verify a signature,
because a manifest outlives its key, but it checked the key only as of
now. It accepted a signature dated after its key expired, after the
key's self-signature or the signing subkey's binding signature expired,
or before the key was made. It now verifies a second time with the
library's clock set to the signature's creation time and refuses the
signature if the key or subkey was not valid then. docs/FORMAT.md
states the rule.

Judgement call: the tests make the refused signatures with the
library's packet API, because openpgp.DetachSign will not sign with a
key that is not valid at its clock's time.

Model: opus-5-5

docs/FORMAT.md

@@ -174,9 +174,18 @@ also refuses a manifest whose `signingPubKey` holds a DSA key or subkey, or any
 secret key or subkey, since checking the self-signatures of a DSA key or the
 numbers of a secret key can take hours when those numbers are very large.
 
-A signing key that has expired does not make the signature bad, because a
-manifest outlives its key. A signature past its own expiry time is not good,
-whether or not the key has expired too.
+A signature is good only if the primary key, and the subkey that made the
+signature, were valid at the signature's creation time under the key validity
+rules of RFC 9580. The key is judged by its current self-signatures and binding
+signatures: for a version 4 key the newest self-signature on its primary user
+ID, for a version 6 key its newest direct-key signature, and for a subkey its
+newest binding signature with the signature embedded in it. So a signature made
+before the newest of those is not good, even if an older one made the key valid
+at that time. The reference implementation checks this by verifying the
+signature a second time with its clock set to the signature's creation time. A
+key that has expired since does not make the signature bad, because a manifest
+outlives its key. A signature past its own expiry time is not good, whether or
+not the key has expired too.
 
 ## Deterministic Serialization