Function sign_channel_jekt_v2 

pub fn sign_channel_jekt_v2(
    private_key: &[u8],
    msgid: &str,
    source_agent: &str,
    source_uid: &str,
    source_channel: &str,
    target_agent: &str,
    ts_secs: i64,
    message: &str,
) -> Option<String>

Sign a cross-channel jekt as source_uid, with that UID’s own LAN private key. None for a key that isn’t 32 bytes, like the other signers.