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/dns/resolver_test.go

@@ -455,3 +455,22 @@ func TestBasicResolver(t *testing.T) {
 		t.Fatal("BasicResolver does not implement Resolver interface")
 	}
 }
+
+// TestBasicResolverQueryIntegration calls Query via the BasicResolver against
+// the local system resolver. Skipped when no local resolver is reachable.
+func TestBasicResolverQueryIntegration(t *testing.T) {
+r := NewBasicResolver()
+server := net.ParseIP("127.0.0.1")
+ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+defer cancel()
+
+// Cover BasicResolver.Query; skip if 127.0.0.1:53 is not available.
+msg, err := r.Query(ctx, server, ".", TypeNS, nil)
+if err != nil {
+t.Logf("skipping (local resolver unavailable): %v", err)
+t.Skip()
+}
+if msg == nil {
+t.Fatal("expected non-nil response from BasicResolver.Query")
+}
+}