fix(iroh): Use valid available IPv6 address, ignoring have_ipv6 (#3419)

## Description

Sometimes net_report says `udp_v6: false`, even though I can clearly
send and receive to/from the other iroh endpoint I want to talk to.
Here are some logs from that situation:
```
2025-07-29T10:15:43.281088Z DEBUG ep{me=68b3f4f4cc}:actor: iroh::net_report: generated report in 728ms report=Report { udp_v4: true, udp_v6: false, mapping_varies_by_dest_ipv4: Some(false), mapping_varies_by_dest_ipv6: None, preferred_relay: Some(RelayUrl("[https://euc1-1.relay.n0.iroh.iroh.link./")](https://euc1-1.relay.n0.iroh.iroh.link./%22))), relay_latency: RelayLatencies { ipv4: {RelayUrl("[https://aps1-1.relay.n0.iroh.iroh.link./"](https://aps1-1.relay.n0.iroh.iroh.link./%22)): 182.188896ms, RelayUrl("[https://euc1-1.relay.n0.iroh.iroh.link./"](https://euc1-1.relay.n0.iroh.iroh.link./%22)): 21.317467ms, RelayUrl("[https://use1-1.relay.n0.iroh.iroh.link./"](https://use1-1.relay.n0.iroh.iroh.link./%22)): 117.29417ms}, ipv6: {}, https: {} }, global_v4: Some(176.199.xxx.xxx:26608), global_v6: None, captive_portal: None }
2025-07-29T10:15:55.933530Z DEBUG ep{me=68b3f4f4cc}:disco_in{node=e10b5cdea1 src=Ip([2a01:599:b2f:a1ad:495a:8b14:xxxx:xxxx]:49154)}:handle_disco{dm=Pong(Pong { tx_id: TransactionId(0xBC381DA98EFD2B20CAE0A96E), ping_observed_addr: Udp([2a02:8071:d85:19c0:9676:965:xxxx:xxxx]:38391) })}:handle_pong{m=Pong { tx_id: TransactionId(0xBC381DA98EFD2B20CAE0A96E), ping_observed_addr: Udp([2a02:8071:d85:19c0:9676:965:xxxx:xxxx]:38391) } src=Udp([2a01:599:b2f:a1ad:495a:8b14:xxxx:xxxx]:49154)}: iroh::_events::holepunched: remote_node=e10b5cdea1 path=[2a01:599:b2f:a1ad:495a:8b14:xxxx:xxxx]:49154 direction="outgoing"
```

Now, the `udp_v6: false` value originally comes from `netwatch`'s
`InterfaceState`, as it sees my (what is now and probably was at the
time fe80-prefixed) link-local address and characterizes it as "not
reachable from the public internet".

But this PR instead ignores that value *iff* we have any
`UdpSendAddr::Valid`, since that would mean it's an address that we've
successfully sent and received from with the particular peer we care
about.

## Change checklist
<!-- Remove any that are not relevant. -->
- [x] Self-review.

Author: matheus23

iroh/src/magicsock.rs

@@ -1879,8 +1879,13 @@ impl Actor {
                         continue;
                     };
                     let is_major = state.is_major_change(&current_netmon_state);
+                    event!(
+                        target: "iroh::_events::link_change",
+                        Level::DEBUG,
+                        ?state,
+                        is_major
+                    );
                     current_netmon_state = state;
-                    trace!("tick: link change {}", is_major);
                     self.msock.metrics.magicsock.actor_link_change.inc();
                     self.handle_network_change(is_major).await;
                 }
@@ -1912,11 +1917,11 @@ impl Actor {
     }
 
     async fn handle_network_change(&mut self, is_major: bool) {
-        debug!("link change detected: major? {}", is_major);
+        debug!(is_major, "link change detected");
 
         if is_major {
             if let Err(err) = self.network_change_sender.rebind() {
-                warn!("failed to rebind transports: {:?}", err);
+                warn!("failed to rebind transports: {err:?}");
             }
 
             #[cfg(not(wasm_browser))]

iroh/src/magicsock/node_map/node_state.rs

@@ -294,22 +294,22 @@ impl NodeState {
         let (best_addr, relay_url) = match self.udp_paths.send_addr(have_ipv6) {
             UdpSendAddr::Valid(addr) => {
                 // If we have a valid address we use it.
-                trace!(%addr, "UdpSendAddr is valid, use it");
+                trace!(%addr, ?have_ipv6, "UdpSendAddr is valid, use it");
                 (Some(*addr), None)
             }
             UdpSendAddr::Outdated(addr) => {
                 // If the address is outdated we use it, but send via relay at the same time.
                 // We also send disco pings so that it will become valid again if it still
                 // works (i.e. we don't need to holepunch again).
-                trace!(%addr, "UdpSendAddr is outdated, use it together with relay");
+                trace!(%addr, ?have_ipv6, "UdpSendAddr is outdated, use it together with relay");
                 (Some(*addr), self.relay_url())
             }
             UdpSendAddr::Unconfirmed(addr) => {
-                trace!(%addr, "UdpSendAddr is unconfirmed, use it together with relay");
+                trace!(%addr, ?have_ipv6, "UdpSendAddr is unconfirmed, use it together with relay");
                 (Some(*addr), self.relay_url())
             }
             UdpSendAddr::None => {
-                trace!("No UdpSendAddr, use relay");
+                trace!(?have_ipv6, "No UdpSendAddr, use relay");
                 (None, self.relay_url())
             }
         };

iroh/src/magicsock/node_map/udp_paths.rs

@@ -132,6 +132,11 @@ impl NodeUdpPaths {
     /// Returns the current UDP address to send on.
     pub(super) fn send_addr(&self, have_ipv6: bool) -> &UdpSendAddr {
         if !have_ipv6 {
+            // If it's a valid address, it doesn't matter if our interface scan determined that we
+            // "probably" don't have IPv6, because we clearly were able to send and receive a ping/pong over IPv6.
+            if matches!(&self.best, UdpSendAddr::Valid(_)) {
+                return &self.best;
+            }
             return &self.best_ipv4;
         }
         &self.best