fix: make embedded NATS opt-in so tests don't hang

Move NATS startup from New() to Start(), so tests that don't use
pubsub never block on server initialization. Add a 10s timeout to
WaitForServer() and skip NATS tests gracefully when unavailable.

pubsub.go

@@ -1,8 +1,9 @@
 package via
 
 // PubSub is an interface for publish/subscribe messaging backends.
-// By default, New() starts an embedded NATS server. Supply a custom
-// implementation via Config(Options{PubSub: yourBackend}) to override.
+// By default, Start() launches an embedded NATS server if no backend
+// has been configured. Supply a custom implementation via
+// Config(Options{PubSub: yourBackend}) to override.
 type PubSub interface {
 	Publish(subject string, data []byte) error
 	Subscribe(subject string, handler func(data []byte)) (Subscription, error)