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.

pubsub.go

@@ -0,0 +1,14 @@
+package via
+
+// PubSub is an interface for publish/subscribe messaging backends.
+// The vianats sub-package provides an embedded NATS implementation.
+type PubSub interface {
+	Publish(subject string, data []byte) error
+	Subscribe(subject string, handler func(data []byte)) (Subscription, error)
+	Close() error
+}
+
+// Subscription represents an active subscription that can be manually unsubscribed.
+type Subscription interface {
+	Unsubscribe() error
+}