feat: add FileStore 6-way permutation tests (Go testFileStoreAllPermutations parity)

Port 20 tests across the {NoCipher, ChaCha, Aes} x {NoCompression, S2Compression}
matrix from Go's testFileStoreAllPermutations (filestore_test.go:55), yielding 120
total Theory executions in FileStorePermutationTests.cs.

Also fix PushFrame.Subject property missing compilation error that was blocking
the test build (JetStreamConsumerDeliveryEdgeTests.cs:119).

Tests covered (each runs 6x):
  Store_and_load_basic                       TestFileStoreBasics:86
  Store_multiple_messages_load_by_sequence   TestFileStoreBasics:86
  LoadLastBySubject_returns_most_recent_for_subject
  Remove_single_message_updates_state        TestFileStoreBasics:129
  Purge_clears_all_messages                  TestFileStorePurge:710
  TrimToMaxMessages_enforces_limit           TestFileStoreMsgLimitBug:518
  Block_rotation_when_exceeding_block_size   TestFileStoreAndRetrieveMultiBlock:1527
  GetState_returns_correct_counts            TestFileStoreBasics:104
  Snapshot_and_restore_round_trip            TestFileStoreSnapshot:1799
  ListAsync_returns_ordered_messages         TestFileStoreTimeStamps:683
  MaxAge_prunes_expired_messages             TestFileStoreAgeLimit:616
  Recovery_after_reopen_preserves_messages   TestFileStoreBasicWriteMsgsAndRestore:181
  Large_payload_store_and_load               64 KiB random payload variant
  Multiple_subjects_filter_by_subject        TestFileStoreBasics multi-subject
  Sequential_writes_maintain_ordering        TestFileStoreSelectNextFirst:304
  Store_creates_files_on_disk                disk-presence variant
  Write_and_read_same_block                  TestFileStoreWriteAndReadSameBlock:1510
  Stored_messages_have_non_decreasing_timestamps TestFileStoreTimeStamps:683
  Remove_out_of_order_collapses_first_seq    TestFileStoreCollapseDmap:1561
  Snapshot_after_removes_preserves_remaining TestFileStoreSnapshot:1904

src/NATS.Server/JetStream/Consumers/PushConsumerEngine.cs

@@ -57,4 +57,11 @@ public sealed class PushFrame
     public bool IsHeartbeat { get; init; }
     public StoredMessage? Message { get; init; }
     public DateTime AvailableAtUtc { get; init; } = DateTime.UtcNow;
+
+    /// <summary>
+    /// The NATS subject of the delivered message. Populated for data frames;
+    /// empty string for heartbeat and flow-control frames.
+    /// Mirrors the Go server's deliver-subject routing (consumer.go).
+    /// </summary>
+    public string Subject => Message?.Subject ?? string.Empty;
 }