feat: implement DNS server fingerprinting (HAN-384)

- Add internal/fingerprint package with Fingerprinter type
  - Sends version.bind CHAOS TXT query to each server
  - Caches results per IP to avoid redundant queries
  - Concurrent batch fingerprinting via FingerprintAll
  - Gracefully handles non-responding servers (returns empty string)
  - Injectable exchange function for testability

- Wire fingerprinting into RunTraversal
  - Triggered when both ShowVersions and ShowServers are enabled
  - Collects unique server IPs from traversal results
  - Stores results in output.Config.Fingerprints before WriteSummary

- Display versions in text output (writeServers)
  - Appends version string after IP when ShowVersions is true
  - No output change when version is unknown

- Include version in JSON output (jsonServer.Version field)
  - omitempty: field absent when version is unknown

- Add comprehensive fingerprint tests (11 test cases)

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

internal/output/runner.go

@@ -3,7 +3,9 @@ package output
 import (
 	"context"
 	"fmt"
+	"net"
 
+	"github.com/hits/ExploreDNS/internal/fingerprint"
 	"github.com/hits/ExploreDNS/internal/traverse"
 )
 
@@ -25,6 +27,13 @@ func RunTraversal(ctx context.Context, traverser *traverse.Traverser, cfg *Confi
 		return results, err
 	}
 
+	// Fingerprint servers when both ShowVersions and ShowServers are enabled.
+	// Gating on ShowServers avoids unnecessary network calls when versions
+	// would not be displayed anyway.
+	if cfg.ShowVersions && cfg.ShowServers {
+		cfg.Fingerprints = fingerprint.New().FingerprintAll(ctx, collectUniqueServerIPs(results))
+	}
+
 	if err := formatter.WriteSummary(results); err != nil {
 		return results, err
 	}
@@ -34,3 +43,20 @@ func RunTraversal(ctx context.Context, traverser *traverse.Traverser, cfg *Confi
 
 	return results, nil
 }
+
+// collectUniqueServerIPs returns the set of unique server IPs seen in results.
+func collectUniqueServerIPs(results []traverse.TraversalResult) []net.IP {
+	seen := make(map[string]bool)
+	var ips []net.IP
+	for _, r := range results {
+		if r.Response == nil || r.Response.Server == nil {
+			continue
+		}
+		key := r.Response.Server.String()
+		if !seen[key] {
+			seen[key] = true
+			ips = append(ips, r.Response.Server)
+		}
+	}
+	return ips
+}