Implement fast userspace multihop mesh routing

Replace the one-intermediate-peer relay with protocol-scoped connectivity
graphs and precomputed shortest-path/ECMP forwarding snapshots. Independent
transport readers forward opaque transit frames without a plugin or TUN
round trip. Carry source, destination, a bounded hop limit and stable flow
tags; preserve end-to-end WireGuard links across topology changes.

Classify IP flows before encryption and preserve their tags through the
WireGuard pending queue. Add offline four-agent path-change coverage, loop
and isolation tests, and an opt-in release forwarding microbenchmark. Bump
control/data ALPNs while preserving persistent network identities and state.

Also include the pending Windows Mainline idle-timeout fix and its regression
test, using a reproducible vendored dependency patch.

Validation: fmt, workspace Clippy with warnings denied, and 307 release tests
passed. Two pre-existing Windows SQLite wipe failures were excluded; public
DHT and the manual benchmark remain ignored by default. The forwarding
microbenchmark measured 103 ns (64 B) and 202 ns (1280 B) per transit packet,
excluding encryption and socket I/O.

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/3`; both ends and intermediate
+The new framing uses data ALPN `tsunagi/data/4`; 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.