Function append_checked 

fn append_checked(
    broker: &Broker,
    fs: &FileStore,
    block_id: &str,
    data: &[u8],
) -> bool

Append to the pane’s term file, then tell the frontend where the bytes landed (the same blockfile append event a PTY’s output produces). Unlike the PTY path’s handle_append_block_file, which logs and swallows a store error, this says whether the bytes were stored, so the remote offset never moves past output that was lost.