Function build_update_payload 

fn build_update_payload(
    updatetype: &str,
    otype: &str,
    oid: &str,
    obj: Option<Value>,
) -> MuxObjUpdate

JSON shape that gets broadcast as the data payload of a waveobj:update WS event. Matches the shape of the MuxObjUpdate struct in crates/srv/src/backend/obj.rs so the frontend’s existing updateMuxObject handler accepts it without changes.