Relay across any mix of links, one online answer for every client, first claim owns a hostname

A tunnel no longer depends on the kind of link under it. The relay hub keeps
one forwarding table per network instead of one per protocol, the envelope
names the end-to-end protocol, and a neighbour reached over several kinds of
transport is sent to over the best open one. A member in the middle needs no
protocol in common with either end. Which protocol a pair runs is settled by
the lower endpoint id's announced order, so both ends agree without asking.
A direct link that comes up later is shorter than the path through others, so
traffic moves to it with the same logical link.

The agent now answers "is it online" once: a live control session or any data
path. Members and peers report how they are reached (direct over a named
transport, relayed through a named member, or no data path yet), and `tsng
status` and the tray both read that instead of a session list and a WireGuard
handshake respectively.

A hostname belongs to whoever claimed it first. A claim carries the time its
author first claimed the name, signed with the record, and replicas order by
(time, id) so every member names the same holder. A later claimant keeps
working by address, is not found by the name, and is told so in its status,
`tsng status`, the tray and a HostnameTaken event.

Wire change: control and data ALPN 5, control PROTOCOL_VERSION 5, envelope
version 2, signing domain v3, state schema 7, local control protocol 18.
Upgrade every member together.

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

AGENTS.md

@@ -59,8 +59,10 @@ Keep these separate. Crossing them is the main thing to review for.
   other peers goes link in, link out: it is never written to the middle's
   interface or decrypted there. Multihop routing has a bounded hop limit;
   equal-cost next hops are chosen per flow. Reachability is first-hand and
-  volatile: each authenticated member advertises its own protocol-specific
-  links. Build routing tables on topology changes, never per packet. Transit
+  volatile: each authenticated member advertises its own direct links, of
+  whatever kind. A link is a way to reach a neighbour and a protocol is an
+  end-to-end tunnel to a peer; the two meet only in the envelope, so a
+  tunnel never depends on which transports carry it. Build routing tables on topology changes, never per packet. Transit
   must not acquire a routing mutex or wait for a protocol/TUN reader.
 - **Broadcast fanout belongs at local IP ingress.** Participation is local to
   each network, enabled by default, persisted and advertised to authenticated