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/resolver.go

@@ -10,12 +10,16 @@ import (
 	"github.com/miekg/dns"
 )
 
+// Resolver is the interface for sending DNS queries.
+// Implementations may cache, mock, or delegate to the real network.
 type Resolver interface {
 	Query(ctx context.Context, server net.IP, name string, qtype uint16, cfg *QueryConfig) (*dns.Msg, error)
 }
 
+// BasicResolver implements Resolver by issuing real UDP/TCP DNS queries.
 type BasicResolver struct{}
 
+// NewBasicResolver returns a BasicResolver ready for use.
 func NewBasicResolver() *BasicResolver {
 	return &BasicResolver{}
 }
@@ -40,6 +44,10 @@ func (e *cacheEntry) expired() bool {
 	return time.Now().After(e.expireAt)
 }
 
+// CachingResolver wraps another Resolver and caches successful responses.
+// Responses are evicted based on the minimum DNS TTL in the message, with a
+// configurable fallback default TTL for zero-TTL responses.
+// All operations are safe for concurrent use.
 type CachingResolver struct {
 	inner      Resolver
 	mu         sync.RWMutex
@@ -47,6 +55,9 @@ type CachingResolver struct {
 	defaultTTL time.Duration
 }
 
+// NewCachingResolver creates a CachingResolver wrapping inner.
+// When inner is nil, a BasicResolver is used.
+// Optional opts may be supplied to customise the default TTL (see WithDefaultTTL).
 func NewCachingResolver(inner Resolver, opts ...CachingResolverOption) *CachingResolver {
 	if inner == nil {
 		inner = NewBasicResolver()
@@ -119,6 +130,7 @@ func (cr *CachingResolver) store(key cacheKey, msg *dns.Msg) {
 	cr.mu.Unlock()
 }
 
+// Len returns the current number of entries in the cache, including expired ones.
 func (cr *CachingResolver) Len() int {
 	cr.mu.RLock()
 	n := len(cr.cache)
@@ -126,12 +138,14 @@ func (cr *CachingResolver) Len() int {
 	return n
 }
 
+// Clear removes all entries from the cache.
 func (cr *CachingResolver) Clear() {
 	cr.mu.Lock()
 	cr.cache = make(map[cacheKey]*cacheEntry)
 	cr.mu.Unlock()
 }
 
+// PurgeExpired removes all expired entries from the cache and returns the count removed.
 func (cr *CachingResolver) PurgeExpired() int {
 	cr.mu.Lock()
 	count := 0
@@ -145,12 +159,15 @@ func (cr *CachingResolver) PurgeExpired() int {
 	return count
 }
 
+// CachingResolverOption is a functional option for NewCachingResolver.
 type CachingResolverOption func(*cachingResolverConfig)
 
 type cachingResolverConfig struct {
 	defaultTTL time.Duration
 }
 
+// WithDefaultTTL sets the TTL used for cache entries when the DNS response
+// contains no TTL information (or when TTL is zero).
 func WithDefaultTTL(d time.Duration) CachingResolverOption {
 	return func(c *cachingResolverConfig) {
 		c.defaultTTL = d