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/json.go

@@ -44,8 +44,9 @@ type jsonResult struct {
 }
 
 type jsonServer struct {
-	Name string   `json:"name"`
-	IPs  []string `json:"ips"`
+	Name    string   `json:"name"`
+	IPs     []string `json:"ips"`
+	Version string   `json:"version,omitempty"`
 }
 
 type jsonSummary struct {
@@ -104,10 +105,16 @@ func (f *jsonFormatter) WriteSummary(results []traverse.TraversalResult) error {
 	if f.cfg.ShowServers {
 		servers := collectServers(results)
 		for name, ips := range servers {
-			f.payload.Servers = append(f.payload.Servers, jsonServer{
-				Name: name,
-				IPs:  ips,
-			})
+			srv := jsonServer{Name: name, IPs: ips}
+			if f.cfg.ShowVersions && f.cfg.Fingerprints != nil {
+				for _, ip := range ips {
+					if v := f.cfg.Fingerprints[ip]; v != "" {
+						srv.Version = v
+						break
+					}
+				}
+			}
+			f.payload.Servers = append(f.payload.Servers, srv)
 		}
 	}