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>

docs/exit-node.md

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+# Exit nodes
+
+A member can offer itself as an **exit node** in a network. Other members can
+then send *all* their internet traffic through it, the way a commercial VPN
+or Tailscale's exit nodes work. Linux only for now.
+
+## Using it
+
+The offering side, per network, off by default:
+
+```sh
+tsunagi network exit-node <network> on      # or off
+```
+
+Rules are installed even when the kernel does not forward yet, and `status`
+says so. Turning forwarding on is left to you, on purpose:
+
+```sh
+sudo sysctl -w net.ipv4.ip_forward=1
+```
+
+The using side:
+
+```sh
+tsunagi exit-node                  # who offers one
+tsunagi exit-node <name|id>        # send everything through it
+tsunagi exit-node off
+```
+
+The tray does the same: a checkbox on the network tile to offer, and an icon on
+each device in the devices window that offers one, with a confirmation.
+
+One device has one default route, so it uses **one** exit node at a time;
+choosing another replaces the first. Offering is per network, so a device can
+be an exit node in one network and not in another.
+
+## How it works
+
+- An offering agent says so in its control-plane `Announce` (`exit_node`), and
+  only once its rules are in place. Members see which peers offer one.
+- **Server:** `iptables` masquerades the overlay range of that network out of
+  any other interface and allows forwarding for it. Rules carry the tag
+  `tsunagi-exit:<interface>:<range>` and are removed when the offer ends, the
+  network stops, and when the agent exits. Only networks that offer have
+  rules, so the others' traffic is not forwarded.
+- **Client:** policy routing, with no change to the main table. Table `28787`
+  holds `default dev <interface>`, and three rules sit after Tailscale's:
+  `5280 uidrange <agent uid> lookup main` (the agent's own connections never
+  go through the tunnel, which would be a loop), `5290 lookup main
+  suppress_prefixlength 0` (LAN and other specific routes keep working) and
+  `5300 lookup 28787`.
+- Packets for addresses outside every overlay range are sent to the exit node
+  by the router; the exit node's replies are accepted from any source. A member
+  that is not your exit node cannot do that.
+
+## What it does not do
+
+- **The agent must run as a system service** (its own user). The uid exemption
+  would otherwise cover everything you run. A per-user agent refuses to use an
+  exit node and says why.
+- **If the exit node disappears the rules stay** and traffic is dropped until
+  it returns or you run `tsunagi exit-node off`. This is deliberate: falling
+  back to the direct path would leak traffic nobody agreed to send in the
+  clear. The agent logs it once, `status` and the tray say so loudly.
+- IPv6 and DNS are untouched. IPv6 traffic still leaves the ordinary way.
+- Kernel forwarding is only read, never changed.
+
+## Trust
+
+Anyone who knows the network's secret can pick your exit node if you offer
+one, and you carry what they send: it leaves from your address and is yours to
+answer for. The exit node sees every destination and, for plain traffic, its
+contents. Choosing an exit node means trusting its owner completely. Offer one
+only in networks whose members you trust, and only from a machine you are
+willing to have that traffic come from.