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/cache.go

@@ -8,6 +8,13 @@ import (
 	miekgdns "github.com/miekg/dns"
 )
 
+// InfoCache is a two-level (parent/child) concurrent cache for NS records and
+// glue addresses discovered during traversal.
+//
+// Lookups walk the parent chain: a child cache falls back to its parent when
+// no local entry is found.  Writes always go to the local cache, never to the
+// parent.  This makes it safe to give sibling branches separate child caches
+// that share the root cache read-only in fast mode.
 type InfoCache struct {
 	parent *InfoCache
 	mu     sync.RWMutex
@@ -15,6 +22,8 @@ type InfoCache struct {
 	glue   map[string][]net.IP
 }
 
+// NewInfoCache creates an InfoCache with an optional parent.
+// Pass nil for a root-level cache with no parent.
 func NewInfoCache(parent *InfoCache) *InfoCache {
 	return &InfoCache{
 		parent: parent,
@@ -23,6 +32,8 @@ func NewInfoCache(parent *InfoCache) *InfoCache {
 	}
 }
 
+// StoreNS records the nameserver names for zone in the local cache.
+// Duplicate names within a zone are deduplicated.
 func (c *InfoCache) StoreNS(zone string, nameservers []string) {
 	if len(nameservers) == 0 {
 		return
@@ -40,6 +51,8 @@ func (c *InfoCache) StoreNS(zone string, nameservers []string) {
 	c.mu.Unlock()
 }
 
+// LookupNS returns the cached nameserver names for zone,
+// walking the parent chain when no local entry is found.
 func (c *InfoCache) LookupNS(zone string) []string {
 	zone = normalize(zone)
 	if names := c.localNS(zone); len(names) > 0 {
@@ -63,6 +76,8 @@ func (c *InfoCache) localNS(zone string) []string {
 	return result
 }
 
+// StoreGlue records the IP addresses for a nameserver hostname in the local cache.
+// Duplicate addresses are deduplicated.
 func (c *InfoCache) StoreGlue(name string, addrs []net.IP) {
 	if len(addrs) == 0 {
 		return
@@ -80,6 +95,8 @@ func (c *InfoCache) StoreGlue(name string, addrs []net.IP) {
 	c.mu.Unlock()
 }
 
+// LookupGlue returns the cached IP addresses for a nameserver hostname,
+// walking the parent chain when no local entry is found.
 func (c *InfoCache) LookupGlue(name string) []net.IP {
 	name = normalize(name)
 	if addrs := c.localGlue(name); len(addrs) > 0 {
@@ -103,16 +120,20 @@ func (c *InfoCache) localGlue(name string) []net.IP {
 	return result
 }
 
+// Child creates a new InfoCache that inherits from c.
+// The child reads from c when a local lookup misses, but never writes to c.
 func (c *InfoCache) Child() *InfoCache {
 	return NewInfoCache(c)
 }
 
+// NSCount returns the number of zone→nameservers entries in the local cache.
 func (c *InfoCache) NSCount() int {
 	c.mu.RLock()
 	defer c.mu.RUnlock()
 	return len(c.ns)
 }
 
+// GlueCount returns the number of nameserver→addresses entries in the local cache.
 func (c *InfoCache) GlueCount() int {
 	c.mu.RLock()
 	defer c.mu.RUnlock()