Function run_su_exec 

pub async fn run_su_exec(
    selection: HostSelection,
    command: &str,
    config_override: Option<PathBuf>,
    format: OutputFormat,
    json: bool,
    opts: ExecOptions,
) -> Result<()>

Runs a command via su - one-shot (consumes su_password).

Workload: I/O-bound SSH. Multi-host uses crate::concurrency::map_bounded.