A fast, modern network bandwidth testing tool with TUI.

Features:
- Live TUI with real-time throughput graphs and per-stream stats
- Server dashboard - xfr serve --tui for monitoring active tests
- Multi-client server - handle multiple simultaneous tests
- TCP and UDP with configurable bitrate and parallel streams
- Bidirectional testing - measure upload and download simultaneously
- Multiple output formats - plain text, JSON, JSON streaming, CSV
- Result comparison - xfr diff to detect performance regressions
- LAN discovery - find xfr servers with mDNS (xfr discover)
- Prometheus metrics - export stats for monitoring dashboards
- Config file - save defaults in ~/.config/xfr/config.toml
- Environment variables - XFR_PORT, XFR_DURATION overrides