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>

Makefile

@@ -3,7 +3,7 @@ BUILD_DIR=bin
 GO=go
 GOFLAGS=-v
 
-.PHONY: build test lint clean
+.PHONY: build test lint clean cover
 
 build:
 	$(GO) build $(GOFLAGS) -o $(BUILD_DIR)/$(BINARY_NAME) ./cmd/exploredns
@@ -11,9 +11,13 @@ build:
 test:
 	$(GO) test -v -race -coverprofile=coverage.out ./...
 
+cover: test
+	$(GO) tool cover -func=coverage.out
+	$(GO) tool cover -html=coverage.out -o coverage.html
+
 lint:
 	$(GO) vet ./...
 
 clean:
 	rm -rf $(BUILD_DIR)
-	rm -f coverage.out
+	rm -f coverage.out coverage.html