Implement referral resolution for nameserver names without glue records

- Add Resolve method to Referral struct for sub-traversal from root
- Add ResolveNS method to Traverser for nameserver name resolution
- Add CircularReferralError and UnresolvableNameserverError types
- Add visited map tracking to detect circular referral chains
- Add depth tracking to prevent infinite loops
- Integrate referral resolution into processReferral
- Add unit tests for new error types
- Handle edge cases: circular referrals, unresolvable nameservers, max depth

Co-authored-by: multica-agent <github@multica.ai>

internal/traverse/referral_test.go

@@ -109,3 +109,51 @@ func TestResolutionStateString(t *testing.T) {
 		})
 	}
 }
+
+func TestCircularReferralError(t *testing.T) {
+	err := &CircularReferralError{
+		Name:  "ns.example.com.",
+		Chain: []string{"ns1.example.com.", "ns2.example.com."},
+	}
+
+	expected := "circular referral detected for ns.example.com.: [ns1.example.com. ns2.example.com.]"
+	if err.Error() != expected {
+		t.Errorf("Error() = %q, want %q", err.Error(), expected)
+	}
+}
+
+func TestUnresolvableNameserverError(t *testing.T) {
+	err := &UnresolvableNameserverError{
+		Name:   "ns.example.com.",
+		Reason: "NXDOMAIN",
+	}
+
+	expected := "unresolvable nameserver ns.example.com.: NXDOMAIN"
+	if err.Error() != expected {
+		t.Errorf("Error() = %q, want %q", err.Error(), expected)
+	}
+}
+
+func TestGetVisitedNames(t *testing.T) {
+	visited := map[string]bool{
+		"ns1.example.com.": true,
+		"ns2.example.com.": true,
+		"ns3.example.com.": true,
+	}
+
+	names := getVisitedNames(visited)
+	if len(names) != 3 {
+		t.Errorf("got %d names, want 3", len(names))
+	}
+
+	seen := make(map[string]bool)
+	for _, name := range names {
+		if seen[name] {
+			t.Errorf("duplicate name: %s", name)
+		}
+		seen[name] = true
+		if !visited[name] {
+			t.Errorf("unexpected name: %s", name)
+		}
+	}
+}