Port IStreamStore, IConsumerStore, StoreMsg, StreamState, SimpleState, ConsumerState, FileStoreConfig, StoreCipher, StoreCompression types. Rename Models.StreamState → ApiStreamState to avoid namespace conflict.
Stream lifecycle, publish/ack, consumer delivery, retention policy, API endpoints, cluster formation, and leader failover tests ported from Go nats-server reference. 1006 total tests passing.