Flushing output and reporting diagnostics

Use write_stderr(text) for diagnostics. It returns the same character-count result as write_stdout. flush_stdout() and flush_stderr() return Result[(), IoError]. Flush when the caller needs to observe buffered-write errors or when displaying a prompt; a flush does not promise disk durability.

def report() -> Result[(), IoError]:
    write_stdout("ready")?
    flush_stdout()?
    Ok(())

def main() -> Result[(), IoError]:
    report()?
    print("")?
    Ok(())

ready