[client] Filter link-local and multicast from network addresses

Skip IPv6 link-local and multicast addresses when building the peer
network_addresses list on non-iOS platforms, matching the existing iOS
behavior. A flapping NIC's link-local address otherwise churns the peer
meta on every interface up/down.

client/system/network_addr.go

@@ -46,7 +46,9 @@ func toNetworkAddress(address net.Addr, mac string) (NetworkAddress, bool) {
 	if !ok {
 		return NetworkAddress{}, false
 	}
-	if ipNet.IP.IsLoopback() {
+	// Skip link-local and multicast: they carry no routable peer info and the
+	// IPv6 link-local of a flapping NIC churns the meta on every up/down.
+	if ipNet.IP.IsLoopback() || ipNet.IP.IsLinkLocalUnicast() || ipNet.IP.IsMulticast() {
 		return NetworkAddress{}, false
 	}
 	prefix, err := netip.ParsePrefix(ipNet.String())

client/system/network_addr_test.go

@@ -0,0 +1,45 @@
+//go:build !ios
+
+package system
+
+import (
+	"net"
+	"testing"
+)
+
+func mustIPNet(t *testing.T, cidr string) *net.IPNet {
+	t.Helper()
+	ip, ipNet, err := net.ParseCIDR(cidr)
+	if err != nil {
+		t.Fatalf("parse %q: %v", cidr, err)
+	}
+	ipNet.IP = ip
+	return ipNet
+}
+
+func TestToNetworkAddress_Filtering(t *testing.T) {
+	const mac = "c8:4b:d6:b6:04:ac"
+
+	tests := []struct {
+		name string
+		cidr string
+		want bool
+	}{
+		{"ipv4 global", "10.65.16.181/23", true},
+		{"ipv6 global", "2620:52:0:4110:102d:6a98:ee75:8b92/64", true},
+		{"ipv4 loopback", "127.0.0.1/8", false},
+		{"ipv6 loopback", "::1/128", false},
+		{"ipv6 link-local", "fe80::871:4c25:23d7:2529/64", false},
+		{"ipv4 link-local", "169.254.1.2/16", false},
+		{"ipv6 multicast", "ff02::1/128", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			_, got := toNetworkAddress(mustIPNet(t, tt.cidr), mac)
+			if got != tt.want {
+				t.Errorf("toNetworkAddress(%s) ok = %v, want %v", tt.cidr, got, tt.want)
+			}
+		})
+	}
+}