OptionalkidIdentifier of the key sign currently uses; the reference signer answers the first 16 hex
characters of the Ed25519 public key. Stamped onto checkpoints so an auditor can tell which
key sealed a row across rotations. Optional: a signer without it seals rows with a null kid.
Signs bytes with the current key — the one Signer.kid names; the reference
adapter signs Ed25519 over the raw bytes.
Accepts still-valid older keys, so a signature made before a key rotation keeps verifying.
Signs bytes and checks signatures, used to vouch for ledger checkpoints.
See
Signer for the contract, the reference adapter, and key rotation.