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/dns/roots.go

@@ -10,12 +10,18 @@ import (
 	"github.com/miekg/dns"
 )
 
+// RootServer holds the name and IP addresses of a DNS root nameserver.
 type RootServer struct {
+	// Name is the FQDN of the root nameserver (e.g. "a.root-servers.net.").
 	Name string
+	// IPv4 holds the IPv4 addresses for the server.
 	IPv4 []net.IP
+	// IPv6 holds the IPv6 addresses for the server.
 	IPv6 []net.IP
 }
 
+// AllIPs returns all IP addresses for the server.
+// When includeAAAA is false, only IPv4 addresses are returned.
 func (rs *RootServer) AllIPs(includeAAAA bool) []net.IP {
 	var ips []net.IP
 	ips = append(ips, rs.IPv4...)
@@ -25,12 +31,19 @@ func (rs *RootServer) AllIPs(includeAAAA bool) []net.IP {
 	return ips
 }
 
+// RootDiscoveryConfig controls how DiscoverRoots selects root servers.
 type RootDiscoveryConfig struct {
-	Server      string
-	AllRoots    bool
+	// Server overrides which root server is used.  An empty string means
+	// auto-select the first root server returned by the system resolver.
+	Server string
+	// AllRoots queries all 13 root servers instead of just one.
+	AllRoots bool
+	// IncludeAAAA includes IPv6 addresses of root servers when true.
 	IncludeAAAA bool
 }
 
+// DefaultRootDiscoveryConfig returns a RootDiscoveryConfig that auto-selects a
+// single IPv4-only root server.
 func DefaultRootDiscoveryConfig() *RootDiscoveryConfig {
 	return &RootDiscoveryConfig{
 		AllRoots:    false,
@@ -38,6 +51,10 @@ func DefaultRootDiscoveryConfig() *RootDiscoveryConfig {
 	}
 }
 
+// DiscoverRoots discovers DNS root servers to use as traversal starting points.
+// When cfg.Server is set, that specific root server is used.
+// When cfg.AllRoots is true, all 13 root servers are returned.
+// Otherwise, a single root server is selected from the system resolver's NS response.
 func DiscoverRoots(ctx context.Context, cfg *RootDiscoveryConfig) ([]RootServer, error) {
 	if cfg == nil {
 		cfg = DefaultRootDiscoveryConfig()