Exit nodes on Windows: use a member as exit node via destination bypass routes

Choosing an exit node on Windows did nothing: the using side was refused,
because Windows has no per-user routing and the agent's own traffic would loop
into the tunnel it carries.

Now the overlay interface gets 0.0.0.0/1 and 128.0.0.0/1 routes (and ::/1,
8000::/1 as an IPv6 blackhole), and every underlay address the agent talks to
directly gets a /32 route through the physical default gateway, marked with
route metric 7373 so stale ones are found and removed. Bypass routes are
written before the halves and removed after them; a failed step removes what
was written. All routes are in the active store.

The addresses come from the runtime: each network runtime hands the interface
the verified direct IPv4 paths of its control connections and the relay hosts
in use (Interface::set_underlay, refreshed every dial round); relay names are
resolved when the rules are applied. ExitHostRules::needs_bypass (true only on
Windows) gates it, so Linux and macOS plans never carry or re-apply on path
changes. ExitHostPlan gains `bypass` and `bypass_hosts`; networks leaving
forget their entry.

Limits: a path that appears after the last apply goes through the tunnel until
the next apply; data-plane-only addresses are not collected separately (the
data links ride the same iroh paths). Nothing was run on Windows.

Checks: cargo fmt --check, cargo clippy --locked --workspace --all-targets
-D warnings and cargo test --locked --workspace --all-targets passed on macOS;
scripts and ordering are unit-tested with an injected runner and resolver, and
Interface::set_underlay with the mock host.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/tsunagi-core/src/agent/mod.rs

@@ -800,6 +800,11 @@ impl Agent {
         // itself stays, because the agent owns it and other networks may
         // still be using it.
         self.inner.routes.remove_network(network_id);
+        if let Some(interface) = self.inner.interface.get() {
+            interface
+                .set_underlay(network_id, Vec::new(), Vec::new())
+                .await;
+        }
         if let Some(interface) = self.inner.interface.get()
             && let Err(err) = interface.sync_addresses().await
         {

crates/tsunagi-core/src/agent/network.rs

@@ -712,6 +712,34 @@ impl Runtime {
         self.start_dials();
         // Last, so it carries what this round learned.
         self.introduce_peers();
+        self.refresh_underlay().await;
+    }
+
+    /// Hands the interface the underlay destinations this network depends
+    /// on: the verified direct IPv4 paths of every control connection, and
+    /// the relays in use. Only hosts that cannot exempt the agent from an
+    /// exit node's default route read it.
+    async fn refresh_underlay(&self) {
+        let Some(interface) = self.params.interface.clone() else {
+            return;
+        };
+        let mut addresses = Vec::new();
+        let mut hosts = Vec::new();
+        for session in self.sessions.values() {
+            for path in snapshot_connection(&session.conn).paths {
+                match path.remote {
+                    PathAddr::Ip(std::net::SocketAddr::V4(socket)) => addresses.push(*socket.ip()),
+                    PathAddr::Relay(url) => hosts.extend(relay_host(&url)),
+                    _ => {}
+                }
+            }
+        }
+        for url in self.params.adapter.snapshot().relay_urls {
+            hosts.extend(relay_host(&url));
+        }
+        interface
+            .set_underlay(self.network_id, addresses, hosts)
+            .await;
     }
 
     fn add_candidate(&mut self, candidate: Candidate) {
@@ -2448,3 +2476,48 @@ mod tests {
         assert_eq!(chosen_restored.copied(), Some("wg"));
     }
 }
+
+/// The host name of a relay URL such as `https://relay.example:443/`.
+fn relay_host(url: &str) -> Option<String> {
+    let rest = url.split_once("://").map_or(url, |(_, rest)| rest);
+    let authority = rest.split(['/', '?', '#']).next()?;
+    let host = authority.rsplit_once('@').map_or(authority, |(_, h)| h);
+    let host = if host.starts_with('[') {
+        host.split_once(']')?.0.trim_start_matches('[')
+    } else {
+        host.split(':').next()?
+    };
+    (!host.is_empty()
+        && host.len() <= 253
+        && host
+            .chars()
+            .all(|c| c.is_ascii_alphanumeric() || c == '.' || c == '-' || c == ':'))
+    .then(|| host.to_ascii_lowercase())
+}
+
+#[cfg(test)]
+mod relay_host_tests {
+    use super::relay_host;
+
+    #[test]
+    fn the_host_is_taken_out_of_a_relay_url() {
+        assert_eq!(
+            relay_host("https://relay.example.com./").as_deref(),
+            Some("relay.example.com.")
+        );
+        assert_eq!(
+            relay_host("https://Euw1-1.relay.iroh.network:443/x").as_deref(),
+            Some("euw1-1.relay.iroh.network")
+        );
+        assert_eq!(
+            relay_host("http://192.0.2.7:3340").as_deref(),
+            Some("192.0.2.7")
+        );
+        assert_eq!(
+            relay_host("https://[2001:db8::1]:443/").as_deref(),
+            Some("2001:db8::1")
+        );
+        assert_eq!(relay_host("https:///").as_deref(), None);
+        assert_eq!(relay_host("https://bad host/").as_deref(), None);
+    }
+}