feat: auto-start embedded NATS server in New()

Pub/sub now works out of the box — New() starts a process-scoped
embedded NATS server with JetStream. The PubSub interface remains
for custom backends via Config(Options{PubSub: ...}).

- Move vianats functionality into nats.go (eliminates circular import)
- Add NATSConn(), JetStream(), EnsureStream(), ReplayHistory[T]() to via
- Delete vianats/ package
- Simplify nats-chatroom and pubsub-crud examples
- Rewrite pubsub tests to use real embedded NATS

nats.go

@@ -0,0 +1,190 @@
+package via
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"os"
+	"sync"
+	"time"
+
+	"github.com/delaneyj/toolbelt/embeddednats"
+	"github.com/nats-io/nats.go"
+)
+
+// defaultNATS is the process-scoped embedded NATS server.
+type defaultNATS struct {
+	server  *embeddednats.Server
+	nc      *nats.Conn
+	js      nats.JetStreamContext
+	cancel  context.CancelFunc
+	dataDir string
+}
+
+var (
+	sharedNATS     *defaultNATS
+	sharedNATSOnce sync.Once
+	sharedNATSErr  error
+)
+
+// getSharedNATS returns a process-level singleton embedded NATS server.
+// The server starts once and is reused across all V instances.
+func getSharedNATS() (*defaultNATS, error) {
+	sharedNATSOnce.Do(func() {
+		sharedNATS, sharedNATSErr = startDefaultNATS()
+	})
+	return sharedNATS, sharedNATSErr
+}
+
+func startDefaultNATS() (dn *defaultNATS, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("nats server panic: %v", r)
+		}
+	}()
+
+	dataDir, err := os.MkdirTemp("", "via-nats-*")
+	if err != nil {
+		return nil, fmt.Errorf("create temp dir: %w", err)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	ns, err := embeddednats.New(ctx, embeddednats.WithDirectory(dataDir))
+	if err != nil {
+		cancel()
+		os.RemoveAll(dataDir)
+		return nil, fmt.Errorf("start embedded nats: %w", err)
+	}
+	ns.WaitForServer()
+
+	nc, err := ns.Client()
+	if err != nil {
+		ns.Close()
+		cancel()
+		os.RemoveAll(dataDir)
+		return nil, fmt.Errorf("connect nats client: %w", err)
+	}
+
+	js, err := nc.JetStream()
+	if err != nil {
+		nc.Close()
+		ns.Close()
+		cancel()
+		os.RemoveAll(dataDir)
+		return nil, fmt.Errorf("init jetstream: %w", err)
+	}
+
+	return &defaultNATS{
+		server:  ns,
+		nc:      nc,
+		js:      js,
+		cancel:  cancel,
+		dataDir: dataDir,
+	}, nil
+}
+
+func (n *defaultNATS) Publish(subject string, data []byte) error {
+	return n.nc.Publish(subject, data)
+}
+
+func (n *defaultNATS) Subscribe(subject string, handler func(data []byte)) (Subscription, error) {
+	sub, err := n.nc.Subscribe(subject, func(msg *nats.Msg) {
+		handler(msg.Data)
+	})
+	if err != nil {
+		return nil, err
+	}
+	return sub, nil
+}
+
+// natsRef wraps a shared defaultNATS as a PubSub. Close is a no-op because
+// the underlying server is process-scoped and outlives individual V instances.
+type natsRef struct {
+	dn *defaultNATS
+}
+
+func (r *natsRef) Publish(subject string, data []byte) error {
+	return r.dn.Publish(subject, data)
+}
+
+func (r *natsRef) Subscribe(subject string, handler func(data []byte)) (Subscription, error) {
+	return r.dn.Subscribe(subject, handler)
+}
+
+func (r *natsRef) Close() error {
+	return nil
+}
+
+// NATSConn returns the underlying NATS connection from the built-in embedded
+// server, or nil if a custom PubSub backend is in use.
+func (v *V) NATSConn() *nats.Conn {
+	if v.defaultNATS != nil {
+		return v.defaultNATS.nc
+	}
+	return nil
+}
+
+// JetStream returns the JetStream context from the built-in embedded server,
+// or nil if a custom PubSub backend is in use.
+func (v *V) JetStream() nats.JetStreamContext {
+	if v.defaultNATS != nil {
+		return v.defaultNATS.js
+	}
+	return nil
+}
+
+// StreamConfig holds the parameters for creating or updating a JetStream stream.
+type StreamConfig struct {
+	Name     string
+	Subjects []string
+	MaxMsgs  int64
+	MaxAge   time.Duration
+}
+
+// EnsureStream creates or updates a JetStream stream matching cfg.
+func EnsureStream(v *V, cfg StreamConfig) error {
+	js := v.JetStream()
+	if js == nil {
+		return fmt.Errorf("jetstream not available")
+	}
+	_, err := js.AddStream(&nats.StreamConfig{
+		Name:      cfg.Name,
+		Subjects:  cfg.Subjects,
+		Retention: nats.LimitsPolicy,
+		MaxMsgs:   cfg.MaxMsgs,
+		MaxAge:    cfg.MaxAge,
+	})
+	return err
+}
+
+// ReplayHistory fetches the last limit messages from subject,
+// deserializing each as T. Returns an empty slice if nothing is available.
+func ReplayHistory[T any](v *V, subject string, limit int) ([]T, error) {
+	js := v.JetStream()
+	if js == nil {
+		return nil, fmt.Errorf("jetstream not available")
+	}
+	sub, err := js.SubscribeSync(subject, nats.DeliverAll(), nats.OrderedConsumer())
+	if err != nil {
+		return nil, err
+	}
+	defer sub.Unsubscribe()
+
+	var msgs []T
+	for {
+		raw, err := sub.NextMsg(200 * time.Millisecond)
+		if err != nil {
+			break
+		}
+		var msg T
+		if json.Unmarshal(raw.Data, &msg) == nil {
+			msgs = append(msgs, msg)
+		}
+	}
+
+	if limit > 0 && len(msgs) > limit {
+		msgs = msgs[len(msgs)-limit:]
+	}
+	return msgs, nil
+}