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/formatter.go

@@ -30,6 +30,10 @@ type Config struct {
 	Quiet              bool
 	Color              bool
 	Debug              int
+
+	// Fingerprints maps server IP strings to their version.bind version strings.
+	// Populated by RunTraversal when ShowVersions and ShowServers are both true.
+	Fingerprints map[string]string
 }
 
 func DefaultConfig() *Config {