Tests for the password-only decryption chain that follows a successful
login (SRP or email OTP, with or without 2FA). The unwrap covers:
password -> KEK (Argon2id) -> masterKey (secretbox) ->
secretKey (secretbox) -> tokenBytes (sealed box) -> base64url token
Each test builds a synthetic AuthorizationResponse using libsodium
directly and asserts unwrapAuth recovers the inputs byte for byte. The
test file also functions as the canonical description of the protocol.
Adds src/auth/types.ts with KeyAttributes, SRPAttributes,
AuthorizationResponse, and LoginChallenge declarations matching the
README's API reference. src/auth/unwrap.ts is the throwing stub; the
real implementation lands next.
