hlverifier package

Package hlverifier is the launch verifier of bridge/PLAN.md §5.1: the ed25519 check of the signers' signatures over a message's Hyperlane checkpoint, under the rule r/hlmailbox keeps beside its signer set — a threshold over the set, or §11.13's two groups (one key of group A and one of group B must sign). It registers itself with the mailbox at deploy and keeps no state: the signers and the rule are the mailbox's, read at every check, so a rotation there is a rotation here. Satisfiable answers whether a signer set could still meet the rule, which is what makes a signer's removal at once or a 7-day wait (§5.3).