Function verify_marker_payload 

pub fn verify_marker_payload(
    payload: &str,
    token: &str,
    session_id: &str,
) -> Option<RenderMarker>

Parse and verify an OSC payload — everything after OSC 8487;.

Total function: hostile payloads yield None, never an error to interpret. Only canonically formatted revisions are accepted, so 1 and 01 cannot both authenticate the same commit, and the MAC compare is constant time.

A trailing BEL or ST is tolerated: a VT parser consumes the terminator before dispatching, so a handler normally passes a payload without one, while a caller scanning raw output with a regex keeps it. Both must work.