docs: comprehensive documentation for ExploreDNS

- Rewrite README.md with overview, features, installation (go install +
  build from source), quick start, full CLI flag reference table,
  output section descriptions, project structure, and development guide

- Add package-level doc comments to all five internal packages:
  config, dns, fingerprint, output, traverse (via doc.go or existing
  package-declaration files)

- Add GoDoc comments on every exported type, constant, function, and
  method across all packages:
  - internal/config: Config struct fields, all Parse*/Default/Validate
  - internal/dns: QueryConfig, Resolver, BasicResolver, CachingResolver,
    ExchangeFunc, RootServer, RootDiscoveryConfig, DecodedResponse,
    ResponseClassification, all exported helpers
  - internal/fingerprint: Fingerprinter, New, NewWithTimeout, Query,
    FingerprintAll
  - internal/traverse: Traverser, TraverserConfig, TraversalResult,
    Referral, ResolutionState, Response, ResponseType, InfoCache,
    Stack, TraverserHooks, EventStage, TraversalEvent, EventHandler,
    CircularReferralError, UnresolvableNameserverError
  - internal/output: Format, Config, Formatter, SummaryStats,
    NewFormatter, AttachHooks, RunTraversal, DefaultConfig,
    ComputeSummary

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

internal/traverse/hooks.go

@@ -1,20 +1,32 @@
 package traverse
 
+// EventStage indicates whether a TraversalEvent is fired at the start or
+// completion of processing a Referral.
 type EventStage int
 
+// Event stage constants.
 const (
-	EventStart EventStage = iota
-	EventComplete
+	EventStart    EventStage = iota // fired when a Referral is about to be queried
+	EventComplete                   // fired after the Response has been produced
 )
 
+// TraversalEvent carries context for a single traversal event delivered to
+// the OnEvent hook.
 type TraversalEvent struct {
-	Stage     EventStage
-	Result    TraversalResult
+	// Stage is EventStart or EventComplete.
+	Stage EventStage
+	// Result carries the Referral and (for EventComplete) the Response.
+	Result TraversalResult
+	// IsResolve is true when the event relates to a nameserver address
+	// resolution sub-traversal rather than the main traversal.
 	IsResolve bool
 }
 
+// EventHandler is the function signature for traversal event callbacks.
 type EventHandler func(TraversalEvent)
 
+// TraverserHooks holds the optional event callback for a Traverser.
+// Assigning OnEvent enables progress and result notifications.
 type TraverserHooks struct {
 	OnEvent EventHandler
 }