Function broadcast_meta_update 

pub(super) fn broadcast_meta_update(
    state: &AppState,
    block_id: &str,
    meta_update: &HashMap<String, Value>,
) -> Result<(), String>

Merge meta_update into block_id’s meta and broadcast a waveobj:update WS event so every subscribed frontend (the drawer, if open) picks it up immediately — the same two steps the setmeta WS handler performs (websocket.rs’s COMMAND_SET_META handler), factored out since ptyshell/* needs the identical pattern from a plain HTTP handler with no WshRpcEngine in scope.