Add --duration, --csv, --quiet flags for automated testing

- --duration N: run client test for N seconds then exit
- --csv <file>: append results to CSV (creates with headers if new)
- --quiet/-q: suppress terminal output (for scripted/machine use)

CSV columns: timestamp, host, port, protocol, direction, duration_s,
  tx_avg_mbps, rx_avg_mbps, tx_bytes, rx_bytes, lost_packets, auth_type

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/bandwidth.rs

@@ -123,6 +123,10 @@ pub fn print_status(
     elapsed: Duration,
     lost_packets: Option<u64>,
 ) {
+    if crate::csv_output::is_quiet() {
+        return;
+    }
+
     let secs = elapsed.as_secs_f64();
     let bits = bytes as f64 * 8.0;
     let bw = if secs > 0.0 { bits / secs } else { 0.0 };