When --port 0 is passed, the OS assigns a random available ephemeral
port (typically 49152-65535). The actual port is resolved after binding
and used for all log output and tunnel forwarding.
This prevents port-scanning attacks against a known fixed port.
Changes:
src/gateway/mod.rs — bind first, extract actual_port from listener,
use actual_port for addr formatting and tunnel.start()
src/main.rs — update CLI help text, conditional log for port=0
8 new edge case tests:
- port_zero_binds_to_random_port
- port_zero_assigns_different_ports
- port_zero_assigns_high_port
- specific_port_binds_exactly
- actual_port_matches_addr_format
- port_zero_listener_accepts_connections
- duplicate_specific_port_fails
- tunnel_gets_actual_port_not_zero
943 tests passing, 0 clippy warnings, cargo fmt clean
c8d4ceee71