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/json.go

@@ -9,9 +9,10 @@ import (
 )
 
 type jsonFormatter struct {
-	cfg     *Config
-	w       io.Writer
-	payload jsonDocument
+	cfg          *Config
+	w            io.Writer
+	payload      jsonDocument
+	fingerprints map[string]string
 }
 
 type jsonDocument struct {
@@ -71,6 +72,10 @@ func newJSONFormatter(cfg *Config, w io.Writer) *jsonFormatter {
 	}
 }
 
+func (f *jsonFormatter) SetFingerprints(fps map[string]string) {
+	f.fingerprints = fps
+}
+
 func (f *jsonFormatter) WriteProgress(event traverse.TraversalEvent) error {
 	if !f.cfg.ShowProgress {
 		return nil
@@ -106,9 +111,9 @@ func (f *jsonFormatter) WriteSummary(results []traverse.TraversalResult) error {
 		servers := collectServers(results)
 		for name, ips := range servers {
 			srv := jsonServer{Name: name, IPs: ips}
-			if f.cfg.ShowVersions && f.cfg.Fingerprints != nil {
+			if f.cfg.ShowVersions && f.fingerprints != nil {
 				for _, ip := range ips {
-					if v := f.cfg.Fingerprints[ip]; v != "" {
+					if v := f.fingerprints[ip]; v != "" {
 						srv.Version = v
 						break
 					}