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/lib.rs

@@ -1,4 +1,5 @@
 pub mod auth;
+pub mod csv_output;
 pub mod bandwidth;
 pub mod client;
 pub mod ecsrp5;