v0.73.0

Added

• userKeySigningSeed, userKeyRecordSigner, userKeySigningKeyMultibase, and
  USER_KEY_SALT (/keys): the user key's Ed25519 signing half, derived from
  the key-agreement half's raw 32-byte secret (HKDF-SHA256, salt
  freewallet/keys/user-key/v1, info signing). Every holder of the user key
  can now sign and verify a record proof with nothing server-served in hand, so
  a record sealed to the vault KAK carries authenticity as well as
  confidentiality.
• recordSignerFromSeed (/keyring): the record signer seam over a raw 32-byte
  Ed25519 seed, for a signing key derived on demand rather than held by an
  agent.
• verified-registry in EVIDENCE (/menders): a registry record whose proof
  verified under the user key the reader holds, before it was decrypted.

Changed

• recordSignerFromSeed, updateKeySigner, and the ladder VM signer take their
  signer from @interop/ed25519-verification-key 8.2.0's didKeySigner()
  instead of each naming the key pair by hand before asking for one.

Removed

• Breaking: UserKey.signingSeed and the client-key record's
  userKey.signingSeed member. The signing half derives from the key-agreement
  secret, so it is neither minted nor stored: mintUserKey returns
  { id, secret }, and encodeClientKeyRecord / decodeClientKeyRecord
  neither write nor read the member.