Function verify_jekt 

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

Verify a claimed signature against the sender’s stored key. Constant-time comparison via Mac::verify_slice (not a manual ==), so this can’t leak timing information about how much of the tag matched.