Function verify_channel_jekt 

pub fn verify_channel_jekt(
    public_key: &[u8],
    msgid: &str,
    source_agent: &str,
    source_channel: &str,
    target_agent: &str,
    ts_secs: i64,
    message: &str,
    sig_b64: &str,
) -> bool

Verify a claimed cross-channel signature against the claimed sender’s published public key (read server-side from the host-global shared registry entry — AgentEntry::jekt_public_key).

Returns false, never panics, for a malformed public key, malformed base64, a wrong-length signature, or a signature that simply doesn’t verify — the same “all of these are identically ‘not verified’” policy verify_lan_jekt and verify_reagent_jekt already document. The caller distinguishes “couldn’t check” from “checked and failed”; this function only answers the second question.