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/output/stats.go

@@ -21,11 +21,18 @@ type answerEntry struct {
 	RRs   []string
 }
 
+// SummaryStats holds the aggregated probability statistics computed from a set
+// of traversal results.
 type SummaryStats struct {
-	ByType  map[string]float64
+	// ByType maps ResponseType strings (e.g. "nxdomain", "servfail") to their
+	// cumulative probability weight.
+	ByType map[string]float64
+	// Answers contains per-RDATA probability statistics for answer results.
 	Answers []answerEntry
 }
 
+// ComputeSummary aggregates terminal TraversalResults into a SummaryStats.
+// Returns nil when there are no terminal results to summarise.
 func ComputeSummary(results []traverse.TraversalResult) *SummaryStats {
 	stats := &SummaryStats{
 		ByType: make(map[string]float64),