Refuse IPv4-mapped prefixes with an error that says so

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

internal/model/addr_test.go

@@ -0,0 +1,28 @@
+package model
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseAddress(t *testing.T) {
+	for _, c := range []struct{ in, want, err string }{
+		{in: "10.0.0.0/8", want: "10.0.0.0/8"},
+		{in: "2001:db8::/32", want: "2001:db8::/32"},
+		{in: "::ffff:10.0.0.1", want: "10.0.0.1"},
+		{in: "10.0.0.1/8", err: "host bits"},
+		{in: "::ffff:0:0/96", err: "IPv4-mapped"},
+		{in: "::ffff:10.0.0.0/104", err: "IPv4-mapped"},
+	} {
+		a, err := ParseAddress(c.in)
+		if c.err != "" {
+			if err == nil || !strings.Contains(err.Error(), c.err) {
+				t.Errorf("%s: error %v, want one saying %q", c.in, err, c.err)
+			}
+			continue
+		}
+		if err != nil || a.String() != c.want {
+			t.Errorf("%s: %q, %v; want %q", c.in, a.String(), err, c.want)
+		}
+	}
+}