Function handle_parallel 

pub fn handle_parallel(
    lines: Vec<String>,
    initial_stdin: Box<dyn Read + Send + 'static>,
    final_stdout: Box<dyn Write + Send + 'static>,
    registry: Arc<CommandRegistry>,
    cancel_token: CancellationToken,
) -> Vec<Result<(), String>>

Execute multiple independent command lines in parallel.

The first line receives initial_stdin / final_stdout; all subsequent lines use io::empty() / io::stdout().