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/config/config.go

@@ -10,6 +10,7 @@ import (
 	"github.com/miekg/dns"
 )
 
+// Sentinel errors returned by validation and parsing functions.
 var (
 	ErrInvalidQueryType     = errors.New("invalid query type")
 	ErrInvalidUDPSize       = errors.New("UDP size must be between 512 and 4096")
@@ -20,22 +21,41 @@ var (
 	ErrInvalidRootServer    = errors.New("invalid root server IP address")
 )
 
+// Config holds all runtime settings for ExploreDNS.
+// Populate it from CLI flags, then call Validate before use.
 type Config struct {
-	QueryType      string
-	RootServer     string
+	// QueryType is the DNS record type to query (e.g. "A", "MX", "TXT").
+	QueryType string
+	// RootServer overrides the root server IP used to begin traversal.
+	// Empty string means auto-discover via the system resolver.
+	RootServer string
+	// AllRootServers queries all 13 DNS root servers instead of one.
 	AllRootServers bool
-	RootAAAA       bool
-	FollowAAAA     bool
-	UDPSize        int
-	AllowTCP       bool
-	AlwaysTCP      bool
-	MaxDepth       int
-	Retries        int
-	Fast           bool
-	Verbose        bool
-	Debug          int
-	Quiet          bool
+	// RootAAAA includes IPv6 addresses of root servers when true.
+	RootAAAA bool
+	// FollowAAAA restricts referral following to AAAA glue records only.
+	FollowAAAA bool
+	// UDPSize is the EDNS0 advertised UDP payload size (512–4096 bytes).
+	UDPSize int
+	// AllowTCP enables TCP fallback when a UDP response is truncated.
+	AllowTCP bool
+	// AlwaysTCP forces all queries over TCP (requires AllowTCP = true).
+	AlwaysTCP bool
+	// MaxDepth limits the traversal depth (1–100).
+	MaxDepth int
+	// Retries is the number of times a failed query is retried (0–10).
+	Retries int
+	// Fast enables the shared-cache fast mode. When true, sibling branches
+	// inherit glue discovered by earlier branches, trading accuracy for speed.
+	Fast bool
+	// Verbose enables verbose output lines.
+	Verbose bool
+	// Debug controls debug verbosity: 0 = off, 1 = app debug, 2 = library debug.
+	Debug int
+	// Quiet suppresses supplementary informational output.
+	Quiet bool
 
+	// Output visibility flags — each controls a section of the output.
 	ShowProgress       bool
 	ShowResolves       bool
 	ShowServers        bool
@@ -45,6 +65,8 @@ type Config struct {
 	ShowSummaryResults bool
 }
 
+// ParseQueryType converts a case-insensitive query type string (e.g. "A", "MX")
+// to its numeric DNS type constant.  Returns ErrInvalidQueryType for unknown types.
 func ParseQueryType(s string) (uint16, error) {
 	s = strings.ToUpper(s)
 	switch s {
@@ -71,6 +93,8 @@ func ParseQueryType(s string) (uint16, error) {
 	}
 }
 
+// ParseUDPSize parses a string as a UDP buffer size.
+// Returns ErrInvalidUDPSize if the string is not an integer or is outside 512–4096.
 func ParseUDPSize(s string) (int, error) {
 	var size int
 	if _, err := fmt.Sscanf(s, "%d", &size); err != nil {
@@ -82,6 +106,8 @@ func ParseUDPSize(s string) (int, error) {
 	return size, nil
 }
 
+// ParseMaxDepth parses a string as a traversal depth.
+// Returns ErrInvalidMaxDepth if the string is not an integer or is outside 1–100.
 func ParseMaxDepth(s string) (int, error) {
 	var depth int
 	if _, err := fmt.Sscanf(s, "%d", &depth); err != nil {
@@ -93,6 +119,8 @@ func ParseMaxDepth(s string) (int, error) {
 	return depth, nil
 }
 
+// ParseRetries parses a string as a retry count.
+// Returns ErrInvalidRetries if the string is not an integer or is outside 0–10.
 func ParseRetries(s string) (int, error) {
 	var retries int
 	if _, err := fmt.Sscanf(s, "%d", &retries); err != nil {
@@ -104,6 +132,9 @@ func ParseRetries(s string) (int, error) {
 	return retries, nil
 }
 
+// Validate checks that all Config fields are within their accepted ranges and
+// that flag combinations are valid (e.g. AlwaysTCP requires AllowTCP).
+// Returns the first validation error encountered, or nil.
 func (c *Config) Validate() error {
 	if _, err := ParseQueryType(c.QueryType); err != nil {
 		return err
@@ -128,6 +159,8 @@ func (c *Config) Validate() error {
 	return nil
 }
 
+// GetDomain returns the domain name from the positional CLI arguments.
+// Returns ErrMissingDomain when args is empty.
 func (c *Config) GetDomain(args []string) (string, error) {
 	if len(args) == 0 {
 		return "", ErrMissingDomain
@@ -135,6 +168,9 @@ func (c *Config) GetDomain(args []string) (string, error) {
 	return args[0], nil
 }
 
+// ParseRootServer parses the RootServer field as an IP address.
+// Returns nil, nil when RootServer is empty (auto-discover mode).
+// Returns ErrInvalidRootServer when the string is non-empty but not a valid IP.
 func (c *Config) ParseRootServer() (net.IP, error) {
 	if c.RootServer == "" {
 		return nil, nil
@@ -158,6 +194,8 @@ func ParseDebugLevel(d, dd bool) int {
 	return 0
 }
 
+// PrintUsage writes a grouped help message to stderr, listing every flag with
+// its description.  It is registered as flag.Usage by main.
 func PrintUsage() {
 	fmt.Fprintf(os.Stderr, "ExploreDNS - DNS reconnaissance and exploration tool\n\n")
 	fmt.Fprintf(os.Stderr, "Usage:\n")
@@ -211,6 +249,9 @@ func PrintUsage() {
 	}
 }
 
+// DefaultConfig returns a Config populated with sensible defaults:
+// query type A, UDP size 2048, max depth 20, 2 retries, fast mode on,
+// TCP fallback allowed, and all output sections visible.
 func DefaultConfig() *Config {
 	return &Config{
 		QueryType:          "A",