feat: add embedded NATS pub/sub support on Context

Define PubSub and Subscription interfaces in the core via package with
a vianats sub-package providing the embedded NATS + JetStream
implementation. Expose c.Publish() and c.Subscribe() on Context with
automatic subscription cleanup on session close. Refactor the NATS
chatroom example to use the built-in methods instead of manual
subscription tracking.

vianats/vianats.go

@@ -0,0 +1,78 @@
+// Package vianats provides an embedded NATS server with JetStream as a
+// pub/sub backend for Via applications.
+package vianats
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/delaneyj/toolbelt/embeddednats"
+	"github.com/nats-io/nats.go"
+	"github.com/ryanhamamura/via"
+)
+
+// NATS implements via.PubSub using an embedded NATS server with JetStream.
+type NATS struct {
+	server *embeddednats.Server
+	nc     *nats.Conn
+	js     nats.JetStreamContext
+}
+
+// New starts an embedded NATS server with JetStream enabled and returns a
+// ready-to-use NATS instance. The server stores data in dataDir and shuts
+// down when ctx is cancelled.
+func New(ctx context.Context, dataDir string) (*NATS, error) {
+	ns, err := embeddednats.New(ctx, embeddednats.WithDirectory(dataDir))
+	if err != nil {
+		return nil, fmt.Errorf("vianats: start server: %w", err)
+	}
+	ns.WaitForServer()
+
+	nc, err := ns.Client()
+	if err != nil {
+		ns.Close()
+		return nil, fmt.Errorf("vianats: connect client: %w", err)
+	}
+
+	js, err := nc.JetStream()
+	if err != nil {
+		nc.Close()
+		ns.Close()
+		return nil, fmt.Errorf("vianats: init jetstream: %w", err)
+	}
+
+	return &NATS{server: ns, nc: nc, js: js}, nil
+}
+
+// Publish sends data to the given subject using core NATS publish.
+// JetStream captures messages automatically if a matching stream exists.
+func (n *NATS) Publish(subject string, data []byte) error {
+	return n.nc.Publish(subject, data)
+}
+
+// Subscribe creates a core NATS subscription for real-time fan-out delivery.
+func (n *NATS) Subscribe(subject string, handler func(data []byte)) (via.Subscription, error) {
+	sub, err := n.nc.Subscribe(subject, func(msg *nats.Msg) {
+		handler(msg.Data)
+	})
+	if err != nil {
+		return nil, err
+	}
+	return sub, nil
+}
+
+// Close shuts down the client connection and embedded server.
+func (n *NATS) Close() error {
+	n.nc.Close()
+	return n.server.Close()
+}
+
+// Conn returns the underlying NATS connection for advanced usage.
+func (n *NATS) Conn() *nats.Conn {
+	return n.nc
+}
+
+// JetStream returns the JetStream context for stream configuration and replay.
+func (n *NATS) JetStream() nats.JetStreamContext {
+	return n.js
+}