feat: add FileStore tombstone, TTL & consumer state persistence (Task 2)

Port Go filestore tombstone/deletion tests, consumer state encode/decode,
consumer file store persistence, and message TTL enforcement. Adds
ConsumerStateCodec and ConsumerFileStore implementations.

17 new tests ported from filestore_test.go.

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

@@ -22,4 +22,8 @@ public sealed class FileStoreOptions
     // Enums are defined in AeadEncryptor.cs.
     public StoreCompression Compression { get; set; } = StoreCompression.NoCompression;
     public StoreCipher Cipher { get; set; } = StoreCipher.NoCipher;
+
+    // Go: StreamConfig.MaxMsgsPer — maximum messages per subject (1 = keep last per subject).
+    // Reference: golang/nats-server/server/filestore.go — per-subject message limits.
+    public int MaxMsgsPerSubject { get; set; }
 }