Implement MCNS v1: custom Go DNS server replacing CoreDNS

Replace the CoreDNS precursor with a purpose-built authoritative DNS
server. Zones and records (A, AAAA, CNAME) are stored in SQLite and
managed via synchronized gRPC + REST APIs authenticated through MCIAS.
Non-authoritative queries are forwarded to upstream resolvers with
in-memory caching.

Key components:
- DNS server (miekg/dns) with authoritative zone handling and forwarding
- gRPC + REST management APIs with MCIAS auth (mcdsl integration)
- SQLite storage with CNAME exclusivity enforcement and auto SOA serials
- 30 tests covering database CRUD, DNS resolution, and caching

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/dns/cache_test.go

@@ -0,0 +1,81 @@
+package dns
+
+import (
+	"testing"
+	"time"
+
+	"github.com/miekg/dns"
+)
+
+func TestCacheSetGet(t *testing.T) {
+	c := NewCache()
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+	msg.Answer = append(msg.Answer, &dns.A{
+		Hdr: dns.RR_Header{Name: "example.com.", Rrtype: dns.TypeA, Class: dns.ClassINET, Ttl: 60},
+		A:   []byte{1, 2, 3, 4},
+	})
+
+	c.Set("example.com.", dns.TypeA, dns.ClassINET, msg, 5*time.Second)
+
+	cached := c.Get("example.com.", dns.TypeA, dns.ClassINET)
+	if cached == nil {
+		t.Fatal("expected cached response")
+	}
+	if len(cached.Answer) != 1 {
+		t.Fatalf("got %d answers, want 1", len(cached.Answer))
+	}
+}
+
+func TestCacheMiss(t *testing.T) {
+	c := NewCache()
+
+	cached := c.Get("example.com.", dns.TypeA, dns.ClassINET)
+	if cached != nil {
+		t.Fatal("expected nil for cache miss")
+	}
+}
+
+func TestCacheExpiry(t *testing.T) {
+	c := NewCache()
+
+	msg := new(dns.Msg)
+	msg.SetQuestion("example.com.", dns.TypeA)
+
+	c.Set("example.com.", dns.TypeA, dns.ClassINET, msg, 1*time.Millisecond)
+	time.Sleep(2 * time.Millisecond)
+
+	cached := c.Get("example.com.", dns.TypeA, dns.ClassINET)
+	if cached != nil {
+		t.Fatal("expected nil for expired entry")
+	}
+}
+
+func TestCacheDifferentTypes(t *testing.T) {
+	c := NewCache()
+
+	msgA := new(dns.Msg)
+	msgA.SetQuestion("example.com.", dns.TypeA)
+	c.Set("example.com.", dns.TypeA, dns.ClassINET, msgA, 5*time.Second)
+
+	msgAAAA := new(dns.Msg)
+	msgAAAA.SetQuestion("example.com.", dns.TypeAAAA)
+	c.Set("example.com.", dns.TypeAAAA, dns.ClassINET, msgAAAA, 5*time.Second)
+
+	cachedA := c.Get("example.com.", dns.TypeA, dns.ClassINET)
+	if cachedA == nil {
+		t.Fatal("expected cached A response")
+	}
+
+	cachedAAAA := c.Get("example.com.", dns.TypeAAAA, dns.ClassINET)
+	if cachedAAAA == nil {
+		t.Fatal("expected cached AAAA response")
+	}
+
+	// Different type should not match.
+	cachedMX := c.Get("example.com.", dns.TypeMX, dns.ClassINET)
+	if cachedMX != nil {
+		t.Fatal("expected nil for uncached type")
+	}
+}