feat: define StreamStore/ConsumerStore interfaces from Go store.go

Port IStreamStore, IConsumerStore, StoreMsg, StreamState, SimpleState,
ConsumerState, FileStoreConfig, StoreCipher, StoreCompression types.
Rename Models.StreamState → ApiStreamState to avoid namespace conflict.

src/NATS.Server/JetStream/Storage/IConsumerStore.cs

@@ -0,0 +1,56 @@
+using NATS.Server.JetStream.Models;
+using StorageType = NATS.Server.JetStream.Models.StorageType;
+
+namespace NATS.Server.JetStream.Storage;
+
+// Go: server/store.go:357
+/// <summary>
+/// Persists and retrieves durable consumer state: delivery progress, ack floor,
+/// pending messages, and redelivery counts. One store instance per consumer.
+/// Mirrors Go's ConsumerStore interface.
+/// </summary>
+public interface IConsumerStore
+{
+    // Go: ConsumerStore.SetStarting — initialise the starting stream sequence for a new consumer
+    void SetStarting(ulong sseq);
+
+    // Go: ConsumerStore.UpdateStarting — update the starting sequence after a reset
+    void UpdateStarting(ulong sseq);
+
+    // Go: ConsumerStore.Reset — reset state to a given stream sequence
+    void Reset(ulong sseq);
+
+    // Go: ConsumerStore.HasState — returns true if any persisted state exists
+    bool HasState();
+
+    // Go: ConsumerStore.UpdateDelivered — record a new delivery (dseq=consumer seq, sseq=stream seq,
+    //     dc=delivery count, ts=Unix nanosecond timestamp)
+    void UpdateDelivered(ulong dseq, ulong sseq, ulong dc, long ts);
+
+    // Go: ConsumerStore.UpdateAcks — record an acknowledgement (dseq=consumer seq, sseq=stream seq)
+    void UpdateAcks(ulong dseq, ulong sseq);
+
+    // Go: ConsumerStore.Update — overwrite the full consumer state in one call
+    void Update(ConsumerState state);
+
+    // Go: ConsumerStore.State — return a snapshot of current consumer state
+    ConsumerState State();
+
+    // Go: ConsumerStore.BorrowState — return state without copying (caller must not retain beyond call)
+    ConsumerState BorrowState();
+
+    // Go: ConsumerStore.EncodedState — return the binary-encoded state for replication
+    byte[] EncodedState();
+
+    // Go: ConsumerStore.Type — the storage type backing this store (File or Memory)
+    StorageType Type();
+
+    // Go: ConsumerStore.Stop — flush and close the store without deleting data
+    void Stop();
+
+    // Go: ConsumerStore.Delete — stop the store and delete all persisted state
+    void Delete();
+
+    // Go: ConsumerStore.StreamDelete — called when the parent stream is deleted
+    void StreamDelete();
+}