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>

docs/wireguard.md

@@ -184,7 +184,7 @@ QUIC DATAGRAM frames themselves cannot fragment.
 Reassembly is bounded and expires incomplete packets; one lost fragment loses
 one packet, without blocking unrelated traffic. The logical data payload limit
 is 64 KiB, with the relay envelope subtracted before it reaches the plugin.
-The new framing uses data ALPN `tsunagi/data/4`; both ends and intermediate
+The new framing uses data ALPN `tsunagi/data/5`; both ends and intermediate
 peers need the updated binary. Network identities and saved state do not change.
 See [protocol.md](protocol.md#the-data-plane-protocol) for the wire format.