fix: address code review issues on PR #10 (HAN-384)

- runner.go: store fingerprints in local var, call formatter.SetFingerprints()
  instead of mutating caller's *Config; removes Fingerprints field from Config
- fingerprint.go: use singleflight.Group in Query to coalesce concurrent
  probes for the same IP, eliminating the double-probe race
- output layer: add SetFingerprints(map[string]string) to Formatter interface;
  textFormatter and jsonFormatter each hold their own fingerprints field
- formatter_test.go: add TestTextFormatterWriteSummaryShowsVersions and
  TestJSONFormatterWriteSummaryShowsVersions verifying version output

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/output/formatter_test.go

@@ -150,6 +150,80 @@ func TestJSONFormatterProducesValidOutput(t *testing.T) {
 	}
 }
 
+func TestTextFormatterWriteSummaryShowsVersions(t *testing.T) {
+	ref := traverse.NewReferral("example.com.", dns.TypeA, "com.", 0, 1.0, nil)
+	serverIP := net.ParseIP("198.41.0.4")
+	resp := &traverse.Response{
+		Referral: ref,
+		Server:   serverIP,
+		Type:     traverse.RespAnswer,
+	}
+
+	var buf bytes.Buffer
+	cfg := DefaultConfig()
+	cfg.Color = false
+	cfg.ShowVersions = true
+	cfg.ShowServers = true
+	cfg.ShowResults = false
+	cfg.ShowSummaryResults = false
+	formatter := NewFormatter(cfg, &buf)
+	formatter.SetFingerprints(map[string]string{serverIP.String(): "BIND 9.18.1"})
+
+	if err := formatter.WriteSummary([]traverse.TraversalResult{{Referral: ref, Response: resp}}); err != nil {
+		t.Fatalf("WriteSummary: %v", err)
+	}
+
+	out := buf.String()
+	if !strings.Contains(out, "BIND 9.18.1") {
+		t.Fatalf("expected version string in text output, got %q", out)
+	}
+}
+
+func TestJSONFormatterWriteSummaryShowsVersions(t *testing.T) {
+	ref := traverse.NewReferral("example.com.", dns.TypeA, "com.", 0, 1.0, nil)
+	serverIP := net.ParseIP("198.41.0.4")
+	resp := &traverse.Response{
+		Referral: ref,
+		Server:   serverIP,
+		Type:     traverse.RespAnswer,
+	}
+
+	var buf bytes.Buffer
+	cfg := DefaultConfig()
+	cfg.Format = FormatJSON
+	cfg.Domain = "example.com"
+	cfg.QueryType = "A"
+	cfg.ShowVersions = true
+	cfg.ShowServers = true
+	cfg.ShowResults = false
+	cfg.ShowSummaryResults = false
+	formatter := NewFormatter(cfg, &buf)
+	formatter.SetFingerprints(map[string]string{serverIP.String(): "Unbound 1.17.0"})
+
+	if err := formatter.WriteSummary([]traverse.TraversalResult{{Referral: ref, Response: resp}}); err != nil {
+		t.Fatalf("WriteSummary: %v", err)
+	}
+	if err := formatter.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
+	}
+
+	var payload map[string]any
+	if err := json.Unmarshal(buf.Bytes(), &payload); err != nil {
+		t.Fatalf("invalid json: %v\n%s", err, buf.String())
+	}
+	servers, ok := payload["servers"].([]any)
+	if !ok || len(servers) == 0 {
+		t.Fatalf("expected servers in json output, got %v", payload)
+	}
+	srv, ok := servers[0].(map[string]any)
+	if !ok {
+		t.Fatalf("expected server object, got %T", servers[0])
+	}
+	if srv["version"] != "Unbound 1.17.0" {
+		t.Fatalf("expected version = %q, got %v", "Unbound 1.17.0", srv["version"])
+	}
+}
+
 func TestRunTraversalUsesHooks(t *testing.T) {
 	answerResp := func() *miekgdns.Msg {
 		m := new(miekgdns.Msg)