feat: complete remaining jetstream parity implementation plan

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

@@ -43,6 +43,23 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         return ValueTask.FromResult(msg);
     }
 
+    public ValueTask<StoredMessage?> LoadLastBySubjectAsync(string subject, CancellationToken ct)
+    {
+        var match = _messages.Values
+            .Where(m => string.Equals(m.Subject, subject, StringComparison.Ordinal))
+            .OrderByDescending(m => m.Sequence)
+            .FirstOrDefault();
+        return ValueTask.FromResult(match);
+    }
+
+    public ValueTask<bool> RemoveAsync(ulong sequence, CancellationToken ct)
+    {
+        var removed = _messages.Remove(sequence);
+        if (removed)
+            RewriteDataFile();
+        return ValueTask.FromResult(removed);
+    }
+
     public ValueTask PurgeAsync(CancellationToken ct)
     {
         _messages.Clear();
@@ -52,6 +69,49 @@ public sealed class FileStore : IStreamStore, IAsyncDisposable
         return ValueTask.CompletedTask;
     }
 
+    public ValueTask<byte[]> CreateSnapshotAsync(CancellationToken ct)
+    {
+        var snapshot = _messages
+            .Values
+            .OrderBy(x => x.Sequence)
+            .Select(x => new FileRecord
+            {
+                Sequence = x.Sequence,
+                Subject = x.Subject,
+                PayloadBase64 = Convert.ToBase64String(x.Payload.ToArray()),
+            })
+            .ToArray();
+        return ValueTask.FromResult(JsonSerializer.SerializeToUtf8Bytes(snapshot));
+    }
+
+    public ValueTask RestoreSnapshotAsync(ReadOnlyMemory<byte> snapshot, CancellationToken ct)
+    {
+        _messages.Clear();
+        _last = 0;
+
+        if (!snapshot.IsEmpty)
+        {
+            var records = JsonSerializer.Deserialize<FileRecord[]>(snapshot.Span);
+            if (records != null)
+            {
+                foreach (var record in records)
+                {
+                    var message = new StoredMessage
+                    {
+                        Sequence = record.Sequence,
+                        Subject = record.Subject ?? string.Empty,
+                        Payload = Convert.FromBase64String(record.PayloadBase64 ?? string.Empty),
+                    };
+                    _messages[record.Sequence] = message;
+                    _last = Math.Max(_last, record.Sequence);
+                }
+            }
+        }
+
+        RewriteDataFile();
+        return ValueTask.CompletedTask;
+    }
+
     public ValueTask<StreamState> GetStateAsync(CancellationToken ct)
     {
         return ValueTask.FromResult(new StreamState