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/text.go

@@ -99,7 +99,13 @@ func (f *textFormatter) writeServers(results []traverse.TraversalResult) error {
 
 	for _, name := range names {
 		for _, ip := range servers[name] {
-			if _, err := fmt.Fprintf(f.w, "%*s: %-15s\n", width, name, ip); err != nil {
+			line := fmt.Sprintf("%*s: %-15s", width, name, ip)
+			if f.cfg.ShowVersions {
+				if version, ok := f.cfg.Fingerprints[ip]; ok && version != "" {
+					line += "  " + version
+				}
+			}
+			if _, err := fmt.Fprintln(f.w, line); err != nil {
 				return err
 			}
 		}