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/traverser.go

@@ -11,13 +11,21 @@ import (
 	miekgdns "github.com/miekg/dns"
 )
 
+// TraverserConfig controls the behaviour of a Traverser.
 type TraverserConfig struct {
-	MaxDepth    int
-	QueryType   uint16
-	RootConfig  *dns.RootDiscoveryConfig
+	// MaxDepth caps the traversal depth.  Referrals at or beyond this depth
+	// are rejected and reported as errors.
+	MaxDepth int
+	// QueryType is the DNS record type requested at each step (e.g. dns.TypeA).
+	QueryType uint16
+	// RootConfig controls how root servers are discovered at startup.
+	RootConfig *dns.RootDiscoveryConfig
+	// QueryConfig controls UDP/TCP transport settings for each DNS query.
 	QueryConfig *dns.QueryConfig
-	RootAddrs   []net.IP
-	Hooks       *TraverserHooks
+	// RootAddrs may be supplied directly to skip root discovery.
+	RootAddrs []net.IP
+	// Hooks receives events during traversal (progress, resolve, result).
+	Hooks *TraverserHooks
 	// Fast controls cache sharing across branches. When true (default), child
 	// branches inherit glue discovered by earlier branches via the shared root
 	// cache, trading accuracy for speed. When false, each branch gets a
@@ -26,6 +34,8 @@ type TraverserConfig struct {
 	Fast bool
 }
 
+// DefaultTraverserConfig returns a TraverserConfig with sensible defaults:
+// max depth 20, query type A, fast mode on.
 func DefaultTraverserConfig() *TraverserConfig {
 	return &TraverserConfig{
 		MaxDepth:    DefaultMaxDepth,
@@ -37,11 +47,17 @@ func DefaultTraverserConfig() *TraverserConfig {
 	}
 }
 
+// TraversalResult pairs a Referral (the query that was attempted) with the
+// Response (the outcome of that query).  Response may be nil for referrals
+// that were never processed (e.g. depth-limit rejections).
 type TraversalResult struct {
 	Referral *Referral
 	Response *Response
 }
 
+// Traverser performs iterative DNS traversal from the root down to the target
+// domain, following referrals and CNAME chains.
+// Create one with NewTraverser; call Traverse to run a traversal.
 type Traverser struct {
 	config   *TraverserConfig
 	exchange dns.ExchangeFunc
@@ -50,6 +66,8 @@ type Traverser struct {
 	mu       sync.Mutex
 }
 
+// NewTraverser creates a Traverser using cfg.
+// When cfg is nil, DefaultTraverserConfig is used.
 func NewTraverser(cfg *TraverserConfig) *Traverser {
 	if cfg == nil {
 		cfg = DefaultTraverserConfig()
@@ -62,10 +80,12 @@ func NewTraverser(cfg *TraverserConfig) *Traverser {
 	}
 }
 
+// SetExchange injects a custom exchange function, primarily for testing.
 func (t *Traverser) SetExchange(fn dns.ExchangeFunc) {
 	t.exchange = fn
 }
 
+// SetHooks attaches traversal event hooks to the Traverser.
 func (t *Traverser) SetHooks(hooks *TraverserHooks) {
 	if t.config == nil {
 		t.config = DefaultTraverserConfig()
@@ -73,6 +93,10 @@ func (t *Traverser) SetHooks(hooks *TraverserHooks) {
 	t.config.Hooks = hooks
 }
 
+// Traverse performs an iterative DNS traversal for name, starting from the root
+// servers.  It returns all TraversalResults, including intermediate referrals
+// and terminal outcomes.  Hooks are called for each event during the traversal.
+// The context can be used to cancel a long-running traversal.
 func (t *Traverser) Traverse(ctx context.Context, name string) ([]TraversalResult, error) {
 	name = miekgdns.Fqdn(name)
 
@@ -251,6 +275,9 @@ func (t *Traverser) processReferral(ctx context.Context, ref *Referral, cache *I
 	}
 }
 
+// ResolveNS resolves a nameserver hostname to its IP addresses by performing
+// a fresh iterative traversal for that name, using cache to avoid repeated
+// queries and visited to detect circular referrals.
 func (t *Traverser) ResolveNS(ctx context.Context, nsName string, cache *InfoCache, visited map[string]bool, depth int) ([]net.IP, error) {
 	if cache != nil {
 		if addrs := cache.LookupGlue(nsName); len(addrs) > 0 {