Refactor the verification handling in the crypto crate

The verification logic has a bunch of layers, the bottom layer is a type-safe state machine. We should be able to simplify this, at least for the SAS verification types.

Begin in the crypto crate and trace the verification logic down to the type-safe state machine, focusing on the SAS verification types named in the issue. Before coding, establish which layers are in scope and how verification behavior will be validated; the issue does not name files, tests, or a concrete done condition.

Written by the indexing model from the issue text.