feat: Add embedded NATS JetStream coordination bus

Central now starts an embedded NATS JetStream server as its core
coordination bus. This provides:

- wild-services KV bucket: service registrations from Cloud/Works
- wild-presence KV bucket: node liveness with TTL-based keys
- wild-events stream: events from all sources (24h retention)

NATS is started in main.go before the API and shut down on SIGTERM.
Cloud and Works will connect to Central's NATS (port 4222) to register
services and maintain presence.

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

internal/natsbus/server_test.go

@@ -0,0 +1,151 @@
+package natsbus
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"github.com/nats-io/nats.go/jetstream"
+)
+
+func TestStartAndShutdown(t *testing.T) {
+	s, err := Start(Config{
+		Port:    -1, // random available port
+		DataDir: t.TempDir(),
+	})
+	if err != nil {
+		t.Fatalf("Start failed: %v", err)
+	}
+	defer s.Shutdown()
+
+	if s.ClientURL() == "" {
+		t.Error("expected non-empty client URL")
+	}
+}
+
+func TestBucketsCreated(t *testing.T) {
+	s, err := Start(Config{
+		Port:    -1,
+		DataDir: t.TempDir(),
+	})
+	if err != nil {
+		t.Fatalf("Start failed: %v", err)
+	}
+	defer s.Shutdown()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Verify services bucket exists
+	kv, err := s.js.KeyValue(ctx, BucketServices)
+	if err != nil {
+		t.Fatalf("services bucket not found: %v", err)
+	}
+
+	// Write and read a value
+	_, err = kv.Put(ctx, "test/svc1", []byte(`{"name":"svc1"}`))
+	if err != nil {
+		t.Fatalf("Put failed: %v", err)
+	}
+
+	entry, err := kv.Get(ctx, "test/svc1")
+	if err != nil {
+		t.Fatalf("Get failed: %v", err)
+	}
+
+	if string(entry.Value()) != `{"name":"svc1"}` {
+		t.Errorf("unexpected value: %s", entry.Value())
+	}
+
+	// Verify presence bucket exists with TTL
+	pKV, err := s.js.KeyValue(ctx, BucketPresence)
+	if err != nil {
+		t.Fatalf("presence bucket not found: %v", err)
+	}
+
+	status, err := pKV.Status(ctx)
+	if err != nil {
+		t.Fatalf("presence status failed: %v", err)
+	}
+
+	if status.TTL() != PresenceTTL {
+		t.Errorf("expected TTL %v, got %v", PresenceTTL, status.TTL())
+	}
+}
+
+func TestEventsStream(t *testing.T) {
+	s, err := Start(Config{
+		Port:    -1,
+		DataDir: t.TempDir(),
+	})
+	if err != nil {
+		t.Fatalf("Start failed: %v", err)
+	}
+	defer s.Shutdown()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Publish an event
+	_, err = s.js.Publish(ctx, "wild.events.test.service-started", []byte(`{"service":"my-api"}`))
+	if err != nil {
+		t.Fatalf("Publish failed: %v", err)
+	}
+
+	// Verify stream has the message
+	stream, err := s.js.Stream(ctx, StreamEvents)
+	if err != nil {
+		t.Fatalf("Stream not found: %v", err)
+	}
+
+	info, err := stream.Info(ctx)
+	if err != nil {
+		t.Fatalf("Stream info failed: %v", err)
+	}
+
+	if info.State.Msgs != 1 {
+		t.Errorf("expected 1 message in stream, got %d", info.State.Msgs)
+	}
+}
+
+func TestPresenceTTL(t *testing.T) {
+	tmpDir := t.TempDir()
+
+	// Use a very short TTL for testing
+	origTTL := PresenceTTL
+	// We can't modify the const, but we can verify the bucket was created with the right TTL
+	_ = origTTL
+
+	s, err := Start(Config{
+		Port:    -1,
+		DataDir: tmpDir,
+	})
+	if err != nil {
+		t.Fatalf("Start failed: %v", err)
+	}
+	defer s.Shutdown()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	kv, err := s.js.KeyValue(ctx, BucketPresence)
+	if err != nil {
+		t.Fatalf("presence bucket not found: %v", err)
+	}
+
+	// Write a presence key
+	_, err = kv.Put(ctx, "nodes/test-node", []byte(`{"ip":"192.168.8.60"}`))
+	if err != nil {
+		t.Fatalf("Put failed: %v", err)
+	}
+
+	// Verify it exists
+	entry, err := kv.Get(ctx, "nodes/test-node")
+	if err != nil {
+		t.Fatalf("Get failed: %v", err)
+	}
+
+	if entry.Operation() != jetstream.KeyValuePut {
+		t.Errorf("expected Put operation, got %v", entry.Operation())
+	}
+}