Incremental authenticated decryption for a private staged destination.
Bytes returned by update remain unauthenticated. Delete the staged file unless finish succeeds, then atomically move it into its final location.
Start from authenticated released-client enc metadata and nonce prefix.
enc
Zeroize state and reject further updates.
Cancel unfinished work and release the native stream.
Authenticate the complete stream and return final hashes.
Decrypt the next ciphertext chunk into unauthenticated staged bytes.