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/stack.go

@@ -1,12 +1,18 @@
 package traverse
 
+// DefaultMaxDepth is the maximum traversal depth used when no explicit depth is configured.
 const DefaultMaxDepth = 20
 
+// Stack is a depth-limited LIFO queue of Referrals used to drive iterative
+// DNS traversal.  Pushing a Referral whose Depth exceeds maxDepth fails
+// silently and returns false, preventing unbounded traversal.
 type Stack struct {
 	items    []*Referral
 	maxDepth int
 }
 
+// NewStack creates a Stack with the given maximum depth.
+// When maxDepth is ≤ 0, DefaultMaxDepth is used.
 func NewStack(maxDepth int) *Stack {
 	if maxDepth <= 0 {
 		maxDepth = DefaultMaxDepth
@@ -17,6 +23,8 @@ func NewStack(maxDepth int) *Stack {
 	}
 }
 
+// Push adds r to the stack.  Returns false (and does not add) when r is nil
+// or r.Depth >= maxDepth.
 func (s *Stack) Push(r *Referral) bool {
 	if r == nil {
 		return false
@@ -28,6 +36,7 @@ func (s *Stack) Push(r *Referral) bool {
 	return true
 }
 
+// Pop removes and returns the top Referral, or nil when the stack is empty.
 func (s *Stack) Pop() *Referral {
 	if len(s.items) == 0 {
 		return nil
@@ -38,6 +47,7 @@ func (s *Stack) Pop() *Referral {
 	return item
 }
 
+// Peek returns the top Referral without removing it, or nil when empty.
 func (s *Stack) Peek() *Referral {
 	if len(s.items) == 0 {
 		return nil
@@ -45,14 +55,17 @@ func (s *Stack) Peek() *Referral {
 	return s.items[len(s.items)-1]
 }
 
+// Len returns the current number of items in the stack.
 func (s *Stack) Len() int {
 	return len(s.items)
 }
 
+// MaxDepth returns the configured maximum depth for this stack.
 func (s *Stack) MaxDepth() int {
 	return s.maxDepth
 }
 
+// IsEmpty reports whether the stack has no items.
 func (s *Stack) IsEmpty() bool {
 	return len(s.items) == 0
 }