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/text.go

@@ -11,14 +11,19 @@ import (
 )
 
 type textFormatter struct {
-	cfg *Config
-	w   io.Writer
+	cfg          *Config
+	w            io.Writer
+	fingerprints map[string]string
 }
 
 func newTextFormatter(cfg *Config, w io.Writer) *textFormatter {
 	return &textFormatter{cfg: cfg, w: w}
 }
 
+func (f *textFormatter) SetFingerprints(fps map[string]string) {
+	f.fingerprints = fps
+}
+
 func (f *textFormatter) WriteProgress(event traverse.TraversalEvent) error {
 	if event.Stage != traverse.EventStart {
 		return nil
@@ -101,7 +106,7 @@ func (f *textFormatter) writeServers(results []traverse.TraversalResult) error {
 		for _, ip := range servers[name] {
 			line := fmt.Sprintf("%*s: %-15s", width, name, ip)
 			if f.cfg.ShowVersions {
-				if version, ok := f.cfg.Fingerprints[ip]; ok && version != "" {
+				if version, ok := f.fingerprints[ip]; ok && version != "" {
 					line += "  " + version
 				}
 			}