feat: rework engine and CLI for dnstraverse parity

Port the traversal engine to the Ruby dnstraverse model so behaviour and
output match dns.squish.net:

- dns: single RD=0 query path (RD=1 only for upstream root discovery),
  per-run packet cache, EDNS0 512-fallback with warnings, UDP->TCP on
  truncation; fix --retries 0 and --root-server IP-literal handling;
  drop all hardcoded 127.0.0.1:53 resolvers
- traverse: hierarchical per-branch InfoCache, 7-step response
  classification with the full 10-status vocabulary, bailiwick
  partitioning, strictly-deeper lame-referral rule, refid grammar with
  .0 resolve subtrees and childset digits, per-IP branching at 1/n
  weight, cache-based glue resolution with noglue/loop dead ends, CNAME
  restarts from the deepest cached zone, fast-mode memoization,
  probability aggregation with Ruby-identical stats keys (sums to 1.0)
- output: byte-for-byte reference text format pinned by a golden test,
  reference CLI defaults, working --quiet/--show-X=false, TTY-aware
  colour, deduplicated deterministic JSON
- web: adapt API/SPA to the new engine, SSE events carry refid/status,
  fix subscribe/snapshot duplicate-event race and a statusCls TDZ bug,
  align SPA type list with the backend
- delete the old engine and dead code (net -4,350 lines)

Verified against live runs of the reference Ruby engine across five
domains (answers, NXDOMAIN, null MX, CNAME restart, glueless resolve)
with no divergences beyond the documented typo fixes.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/output/formatter.go

@@ -1,10 +1,11 @@
-// Package output renders ExploreDNS traversal results for human consumption or
-// machine processing.
+// Package output renders ExploreDNS traversal results for human consumption
+// or machine processing.
 //
 // Two formats are supported:
 //
-//   - FormatText — a coloured hierarchical tree (default)
-//   - FormatJSON — a JSON array of traversal results
+//   - FormatText — dnstraverse-style header/progress/results/summary text
+//     (default)
+//   - FormatJSON — a single JSON document with the aggregated results
 //
 // Create a Formatter via NewFormatter and call RunTraversal to drive the
 // traversal engine and stream output incrementally.
@@ -26,9 +27,19 @@ const (
 )
 
 type Config struct {
-	Format             Format
-	Domain             string
-	QueryType          string
+	Format    Format
+	Domain    string
+	QueryType string
+
+	// Engine settings echoed in the header block (bin/dnstraverse).
+	Fast           bool
+	AllRootServers bool
+	UDPSize        int
+	Retries        int
+	MaxDepth       int
+	AllowTCP       bool
+	AlwaysTCP      bool
+
 	ShowProgress       bool
 	ShowResolves       bool
 	ShowServers        bool
@@ -49,22 +60,50 @@ type Config struct {
 func DefaultConfig() *Config {
 	return &Config{
 		Format:             FormatText,
+		Fast:               true,
+		UDPSize:            2048,
+		Retries:            2,
+		MaxDepth:           20,
+		AllowTCP:           true,
 		ShowProgress:       true,
-		ShowResolves:       true,
-		ShowServers:        true,
+		ShowResolves:       false,
+		ShowServers:        false,
 		ShowVersions:       true,
-		ShowAllStats:       true,
+		ShowAllStats:       false,
 		ShowResults:        true,
 		ShowSummaryResults: true,
-		Color:              os.Getenv("NO_COLOR") == "",
+		Color:              ColorEnabled(os.Stdout),
 	}
 }
 
+// ColorEnabled reports whether colour output should be used for w: only when
+// NO_COLOR is unset and w is a terminal.
+func ColorEnabled(w io.Writer) bool {
+	if os.Getenv("NO_COLOR") != "" {
+		return false
+	}
+	f, ok := w.(*os.File)
+	if !ok {
+		return false
+	}
+	info, err := f.Stat()
+	if err != nil {
+		return false
+	}
+	return info.Mode()&os.ModeCharDevice != 0
+}
+
+// Formatter renders traversal progress and the aggregated results. The header
+// is written once before the run, progress arrives via the traverser hooks,
+// and the aggregated leaves and the servers seen arrive once the run
+// completed.
 type Formatter interface {
+	// WriteHeader renders the pre-run header block from the discovered roots
+	// (suppressed entirely by --quiet in text mode).
+	WriteHeader(roots []traverse.StartServer) error
 	WriteProgress(event traverse.TraversalEvent) error
 	WriteResolve(event traverse.TraversalEvent) error
-	WriteResult(result traverse.TraversalResult) error
-	WriteSummary(results []traverse.TraversalResult) error
+	WriteSummary(root *traverse.Referral, servers map[string][]string) error
 	Flush() error
 }
 
@@ -85,6 +124,11 @@ func AttachHooks(cfg *Config, formatter Formatter) *traverse.TraverserHooks {
 	if cfg == nil || formatter == nil {
 		return nil
 	}
+	if !cfg.ShowProgress {
+		// Ruby registers no progress callbacks at all without show-progress;
+		// resolve display additionally requires show-resolves.
+		return nil
+	}
 	logErr := func(context string, err error) {
 		if err != nil && cfg.Debug > 0 {
 			fmt.Fprintf(os.Stderr, "Debug: formatter %s: %v\n", context, err)
@@ -92,15 +136,13 @@ func AttachHooks(cfg *Config, formatter Formatter) *traverse.TraverserHooks {
 	}
 	return &traverse.TraverserHooks{
 		OnEvent: func(event traverse.TraversalEvent) {
-			switch {
-			case event.IsResolve && cfg.ShowResolves:
-				logErr("WriteResolve", formatter.WriteResolve(event))
-			case !event.IsResolve && cfg.ShowProgress:
-				logErr("WriteProgress", formatter.WriteProgress(event))
-			}
-			if event.Stage == traverse.EventComplete && cfg.ShowAllStats {
-				logErr("WriteResult", formatter.WriteResult(event.Result))
+			if event.IsResolve {
+				if cfg.ShowResolves {
+					logErr("WriteResolve", formatter.WriteResolve(event))
+				}
+				return
 			}
+			logErr("WriteProgress", formatter.WriteProgress(event))
 		},
 	}
 }