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/fingerprint/fingerprint.go

@@ -7,6 +7,7 @@ import (
 	"time"
 
 	miekgdns "github.com/miekg/dns"
+	"golang.org/x/sync/singleflight"
 )
 
 const defaultTimeout = 2 * time.Second
@@ -17,6 +18,7 @@ type Fingerprinter struct {
 	mu       sync.Mutex
 	cache    map[string]string
 	timeout  time.Duration
+	group    singleflight.Group
 	exchange func(ctx context.Context, addr string, m *miekgdns.Msg) (*miekgdns.Msg, error)
 }
 
@@ -36,6 +38,8 @@ func NewWithTimeout(timeout time.Duration) *Fingerprinter {
 // Query returns the version string for ip, or "" if the server doesn't
 // respond or doesn't support the version.bind CHAOS query.
 // Results are cached: subsequent calls for the same IP return immediately.
+// Concurrent calls for the same IP are coalesced via singleflight so only
+// one network probe is issued per IP at a time.
 func (f *Fingerprinter) Query(ctx context.Context, ip net.IP) string {
 	key := ip.String()
 
@@ -46,13 +50,15 @@ func (f *Fingerprinter) Query(ctx context.Context, ip net.IP) string {
 	}
 	f.mu.Unlock()
 
-	version := f.probe(ctx, ip)
+	v, _, _ := f.group.Do(key, func() (interface{}, error) {
+		version := f.probe(ctx, ip)
+		f.mu.Lock()
+		f.cache[key] = version
+		f.mu.Unlock()
+		return version, nil
+	})
 
-	f.mu.Lock()
-	f.cache[key] = version
-	f.mu.Unlock()
-
-	return version
+	return v.(string)
 }
 
 // FingerprintAll queries all ips concurrently and returns a map of