Exit nodes: offer one per network, send all traffic through a member

A member can offer itself as an exit node per network (off by default); it is
announced over the control channel (ctrl/4) once its iptables masquerade and
forward rules are in place, with a warning when kernel forwarding is off. A
device picks one member as its exit node via policy routing, with the agent's
own uid exempt from the tunnel; if the exit node disappears the rules stay
and traffic is dropped, loudly reported. CLI: `network exit-node`,
`exit-node`; tray: tile checkbox, warnings and a per-device icon with
confirmation. Linux only.

Co-Authored-By: Claude <noreply@anthropic.com>

README.md

@@ -746,6 +746,7 @@ networks first if the addresses should be freed.
 - [docs/protocol.md](docs/protocol.md) — identity derivation, framing, handshake.
 - [docs/sync-model.md](docs/sync-model.md) — the planned signed-state model and
   what is deliberately not built yet.
+- [docs/exit-node.md](docs/exit-node.md) — exit nodes: sending all traffic through a member.
 - [docs/threat-model.md](docs/threat-model.md) — threat model and known limits.
 - [docs/testing.md](docs/testing.md) — what the suite covers and what it does not.
 - [AGENTS.md](AGENTS.md) — rules for anyone (human or agent) changing this repo.