Function open_bundle 

pub fn open_bundle<'a>(
    sealed: &'a AuthenticatedJoinBundle,
    cluster_secret: &[u8; 32],
    token_hash: &[u8; 32],
) -> Result<&'a [u8], BundleError>

Verify the MAC on sealed and return the inner bundle bytes.

The comparison is constant-time via hmac::Mac::verify_slice.