feat: add comprehensive test suite for ExploreDNS

- Unit tests for all internal packages exceeding 80% coverage:
  - internal/config: 86.2% (ParseMaxDepth, ParseRetries, validation paths)
  - internal/dns: 82.9% (IterativeQueryWithExchange, mock DNS server, roots)
  - internal/fingerprint: 96.4% (unchanged, already excellent)
  - internal/output: 86.7% (formatters, stats, JSON/text output, hooks)
  - internal/traverse: 86.7% (SetHooks, ResolveNS, processReferral,
    ensureRDFalse, resolveGlueViaSystem, newAQuery, Referral.Resolve)

- Integration tests in internal/integration/:
  - End-to-end traversal with mock DNS exchange function
  - Referral chain traversal (root -> TLD -> authoritative)
  - CNAME resolution and loop detection
  - NXDOMAIN and SERVFAIL response handling
  - Max depth enforcement
  - Context cancellation
  - TraverserHooks event delivery

- Mock DNS server helper in internal/dns/roots_test.go using miekg/dns
  (enables deterministic testing without network dependency)

- CI updated with coverage reporting step
- Makefile: added 'cover' target for local HTML coverage reports

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/config/config_test.go

@@ -214,3 +214,76 @@ func TestParseDebugLevel(t *testing.T) {
 		}
 	}
 }
+
+func TestParseMaxDepthValid(t *testing.T) {
+cases := []string{"1", "20", "100"}
+for _, s := range cases {
+v, err := ParseMaxDepth(s)
+if err != nil {
+t.Errorf("ParseMaxDepth(%q) unexpected error: %v", s, err)
+}
+if v < 1 || v > 100 {
+t.Errorf("ParseMaxDepth(%q) = %d, out of range", s, v)
+}
+}
+}
+
+func TestParseMaxDepthInvalid(t *testing.T) {
+cases := []string{"0", "101", "notanumber"}
+for _, s := range cases {
+_, err := ParseMaxDepth(s)
+if err == nil {
+t.Errorf("ParseMaxDepth(%q): expected error", s)
+}
+if !errors.Is(err, ErrInvalidMaxDepth) {
+t.Errorf("ParseMaxDepth(%q): expected ErrInvalidMaxDepth, got %v", s, err)
+}
+}
+}
+
+func TestParseRetriesValid(t *testing.T) {
+cases := []string{"0", "5", "10"}
+for _, s := range cases {
+v, err := ParseRetries(s)
+if err != nil {
+t.Errorf("ParseRetries(%q) unexpected error: %v", s, err)
+}
+if v < 0 || v > 10 {
+t.Errorf("ParseRetries(%q) = %d, out of range", s, v)
+}
+}
+}
+
+func TestParseRetriesInvalid(t *testing.T) {
+cases := []string{"-1", "11", "notanumber"}
+for _, s := range cases {
+_, err := ParseRetries(s)
+if err == nil {
+t.Errorf("ParseRetries(%q): expected error", s)
+}
+if !errors.Is(err, ErrInvalidRetries) {
+t.Errorf("ParseRetries(%q): expected ErrInvalidRetries, got %v", s, err)
+}
+}
+}
+
+func TestValidateBadQueryType(t *testing.T) {
+cfg := DefaultConfig()
+cfg.QueryType = "BOGUS"
+if err := cfg.Validate(); !errors.Is(err, ErrInvalidQueryType) {
+t.Errorf("expected ErrInvalidQueryType, got %v", err)
+}
+}
+
+func TestDefaultConfigIsValid(t *testing.T) {
+cfg := DefaultConfig()
+if cfg.QueryType != "A" {
+t.Errorf("QueryType = %q, want A", cfg.QueryType)
+}
+if cfg.MaxDepth != 20 {
+t.Errorf("MaxDepth = %d, want 20", cfg.MaxDepth)
+}
+if cfg.Retries != 2 {
+t.Errorf("Retries = %d, want 2", cfg.Retries)
+}
+}