ipn/ipnlocal: install IPv6 service addr route (#13252)

This is the equivalent of quad-100, but for IPv6. This is technically
already contained in the Tailscale IPv6 ULA prefix, but that is only
installed when remote peers are visible via control with contained
addrs. The service addr should always be reachable.

Updates #1152

Signed-off-by: Jordan Whited <jordan@tailscale.com>

ipn/ipnlocal/local.go

@@ -4604,6 +4604,9 @@ func (b *LocalBackend) routerConfig(cfg *wgcfg.Config, prefs ipn.PrefsView, oneC
 	if slices.ContainsFunc(rs.LocalAddrs, tsaddr.PrefixIs4) {
 		rs.Routes = append(rs.Routes, netip.PrefixFrom(tsaddr.TailscaleServiceIP(), 32))
 	}
+	if slices.ContainsFunc(rs.LocalAddrs, tsaddr.PrefixIs6) {
+		rs.Routes = append(rs.Routes, netip.PrefixFrom(tsaddr.TailscaleServiceIPv6(), 128))
+	}
 
 	return rs
 }