The native data plane used a random ephemeral UDP port (hole-punched), which a
strict firewall can't pre-open — so remote clients behind one couldn't connect.
Add an optional fixed data port:
- `Punktfunk1Options`/`NativeServe` gain `data_port`; `bind_data_socket` binds the
fixed port (→ direct, no hole-punch) or falls back to a random port + hole-punch
when unset or the fixed port is busy (a concurrent session already holds it).
- `UdpTransport::from_socket`/`from_socket_punch` adopt an already-bound socket, so
the host keeps the SAME data socket from handshake through streaming — no
drop-then-rebind window in which a concurrent session could steal a fixed port.
- `main.rs` wires the CLI flag through to `NativeServe`.
- Firewall docs updated (troubleshooting.md + apt/pacman/bazzite READMEs): control
plane is the fixed UDP 9777; the data plane is a separate random port that usually
needs no rule, with the fixed-port option for strict firewalls.
Unit-tested: default random+hole-punch, and fixed-port-then-fallback-when-busy.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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