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>

go.mod

@@ -2,12 +2,14 @@ module github.com/hits/ExploreDNS
 
 go 1.25.6
 
-require github.com/miekg/dns v1.1.72
+require (
+	github.com/miekg/dns v1.1.72
+	golang.org/x/sync v0.19.0
+)
 
 require (
 	golang.org/x/mod v0.31.0 // indirect
 	golang.org/x/net v0.48.0 // indirect
-	golang.org/x/sync v0.19.0 // indirect
 	golang.org/x/sys v0.39.0 // indirect
 	golang.org/x/tools v0.40.0 // indirect
 )