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.

AGENTS.md

@@ -45,9 +45,11 @@ Keep these separate. Crossing them is the main thing to review for.
   is why the authenticated transport does too.
 - **A relay carries bytes, never packets.** A datagram passed on for two
   other peers goes link in, link out: it is never written to the middle's
-  interface, never decrypted there, and never relayed twice. Reachability
-  that decides a route is first-hand and volatile — a live announcement
-  over the control plane, never a signed record and never second-hand.
+  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
+  must not acquire a routing mutex or wait for a protocol/TUN reader.
 - **Plugins never learn reachability.** An `IpPlugin` is handed a `PacketLink`
   per peer and moves datagrams over it. Addresses, hole punching and relays
   belong to `crates/tsunagi/src/dataplane/transport/`. A plugin announcement says *who*, never