Add typed Publish[T] and Subscribe[T] generic helpers that handle
JSON marshaling, along with vianats.EnsureStream and ReplayHistory
helpers. Refactor nats-chatroom to use the new APIs.
Add pubsub-crud example demonstrating CRUD operations with DaisyUI
toast notifications broadcast to all connected clients via NATS.
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.