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
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