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.

Cargo.lock

@@ -1985,8 +1985,6 @@ checksum = "d3d25b0e0b648a86960ac23b7ad4abb9717601dec6f66c165f5b037f3f03065f"
 [[package]]
 name = "mainline"
 version = "8.0.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d32eaee3dcba6e0bbbefe8bd896a8bd6039d5e74b199c0fe248e9feb547c2a26"
 dependencies = [
  "crc",
  "document-features",
@@ -3915,6 +3913,7 @@ checksum = "e421abadd41a4225275504ea4d6566923418b7f05506fbc9c0fe86ba7396114b"
 name = "tsunagi"
 version = "0.1.0"
 dependencies = [
+ "arc-swap",
  "bytes",
  "caps",
  "data-encoding",