Make the protocol a crate of its own

tsunagi-wg-quic. The line between a protocol and the system level is now
drawn by the compiler: nothing in it can reach into tsunagi beyond what
tsunagi makes public, and it carries its own version — which is not the
version peers compare.

Two things the compiler found the moment the boundary was real. The key
store was reaching into the core's `pub(crate)` file-permission helpers;
those are a legitimate service of the system level, because a protocol
keeping keys on disk has the same obligation the agent does, so they are
public now with that said. And the test harness was about to be copied
into a second crate, which is how two copies start to drift; it is a
`testing` feature of the core instead, which is also what anybody writing
a protocol would need.

The bridges put up while things were moving are gone: the error
conversion between the two levels, and the re-exports of the system
level's types from the protocol crate. Imports now say which level they
come from, which is the point.

One deliberate deviation, stated rather than hidden. The authenticated
transport stayed in the core. Moving it would have meant handing a
protocol the network's keys so it could prove membership itself, and a
plugin that can authenticate on the control plane is a worse trade than
a module boundary is worth. So the core proves who is at the other end
and the protocol owns what is said over it — the same separation, without
the secret crossing.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -86,13 +86,13 @@ It prints its endpoint id and then waits. On the second machine, pass that id:
 Within a few seconds both print something like:
 
 ```text
-  + peer b47c958462 connected over Direct rtt=Some(4.5ms)
-  + data link to b47c958462 for wireguard: Direct via Ip(…), datagram 1382
+  + peer b47c958462 connected over direct rtt=Some(4.5ms)
+  + data link to b47c958462 for wg-quic: direct via 192.0.2.7:41234, datagram 1382
 
 --- status ---
 control: 1 peer(s), 0 dial failure(s), 0 handshake failure(s)
-wireguard: tsunkkcp43lmdje on fd15:1d9e:fa21:f201:…/64 mtu 1280, 1/1 tunnel(s) established
-  4jO4kx9Z fd15:1d9e:fa21:f201:… handshake 3s ago tx=0 rx=0 dropped=0 path=Direct via Ip(…)
+wg-quic: tsun0 on 10.13.37.69/24 mtu 1280, 1/1 tunnel(s) established
+  4jO4kx9Z 10.13.37.237 handshake 3s ago tx=0 rx=0 dropped=0 path=direct via 192.0.2.7:41234
 ```
 
 `1/1 tunnel(s) established` means a real WireGuard handshake completed.
@@ -384,7 +384,6 @@ tests:
 
 ```bash
 cargo run --example two_agents       # control plane only
-cargo run --example wireguard_mesh   # a WireGuard overlay carrying a real packet
 ```
 
 Both run with no privileges and change nothing on the host.
@@ -477,9 +476,9 @@ paths; tests always use temporary directories.
 | `state.sqlite` | device identity, network configuration, hostname | clear error, never reset |
 | `cache.sqlite` | address hints and other recoverable data          | discarded and recreated  |
 
-The WireGuard plugin keeps its own keys in its own `wireguard.sqlite`, wherever
-its configuration points, because plugin keys are neither the iroh identity nor
-the network secret.
+The `wg-quic` protocol keeps its own keys in its own store under `wg-quic/`,
+because a protocol's keys are neither the iroh identity nor the network
+secret.
 
 The command line agent puts everything under the platform's per-user
 directories by default; `--state-dir` and `--cache-dir` override them.