feat: add atomic batch publish engine & versioning support (Tasks 9-10)

- AtomicBatchPublishEngine: stage/commit/rollback semantics for batch publish
- JsVersioning: API level negotiation and stream/consumer metadata
- Fix NormalizeConfig missing AllowAtomicPublish, Metadata, PersistMode copy
- 46 batch publish tests + 67 versioning tests, all passing

src/NATS.Server/JetStream/Publish/JetStreamPublisher.cs

@@ -5,6 +5,10 @@ public sealed class JetStreamPublisher
     private readonly StreamManager _streamManager;
     private readonly PublishPreconditions _preconditions = new();
 
+    // One engine per publisher (stream-scoped in real server; here publisher-scoped).
+    // Go reference: server/jetstream_batching.go streamBatches
+    private readonly AtomicBatchPublishEngine _batchEngine = new();
+
     public JetStreamPublisher(StreamManager streamManager)
     {
         _streamManager = streamManager;
@@ -24,13 +28,19 @@ public sealed class JetStreamPublisher
             return false;
         }
 
+        // --- Atomic batch publish path ---
+        // Go: server/stream.go processInboundMsg — checks batch headers before normal flow.
+        if (!string.IsNullOrEmpty(options.BatchId))
+        {
+            ack = ProcessBatchMessage(stream, subject, payload, options);
+            return true;
+        }
+
+        // --- Normal (non-batch) publish path ---
         var state = stream.Store.GetStateAsync(default).GetAwaiter().GetResult();
         if (!_preconditions.CheckExpectedLastSeq(options.ExpectedLastSeq, state.LastSeq))
         {
-            ack = new PubAck
-            {
-                ErrorCode = 10071,
-            };
+            ack = new PubAck { ErrorCode = 10071 };
             return true;
         }
 
@@ -50,4 +60,114 @@ public sealed class JetStreamPublisher
         _preconditions.TrimOlderThan(stream.Config.DuplicateWindowMs);
         return true;
     }
+
+    // Go: server/stream.go processInboundMsg — batch message handling.
+    private PubAck ProcessBatchMessage(
+        StreamHandle stream,
+        string subject,
+        ReadOnlyMemory<byte> payload,
+        PublishOptions options)
+    {
+        // Stream must have AllowAtomicPublish enabled.
+        // Go: server/stream.go:6351 NewJSAtomicPublishDisabledError
+        if (!stream.Config.AllowAtomicPublish)
+        {
+            return new PubAck
+            {
+                ErrorCode = AtomicBatchPublishErrorCodes.Disabled,
+                Stream = stream.Config.Name,
+            };
+        }
+
+        // BatchSeq must be present (non-zero).
+        // Go: server/stream.go:6371 NewJSAtomicPublishMissingSeqError
+        if (options.BatchSeq == 0)
+        {
+            return new PubAck
+            {
+                ErrorCode = AtomicBatchPublishErrorCodes.MissingSeq,
+                Stream = stream.Config.Name,
+            };
+        }
+
+        // Nats-Expected-Last-Msg-Id is unsupported in batch context.
+        // Go: server/stream.go:6584 NewJSAtomicPublishUnsupportedHeaderBatchError
+        if (!string.IsNullOrEmpty(options.ExpectedLastMsgId))
+        {
+            return new PubAck
+            {
+                ErrorCode = AtomicBatchPublishErrorCodes.UnsupportedHeader,
+                Stream = stream.Config.Name,
+            };
+        }
+
+        var commitValue = options.BatchCommit;
+        var isCommit = !string.IsNullOrEmpty(commitValue);
+
+        // Validate commit value immediately if present.
+        if (isCommit && commitValue is not ("1" or "eob"))
+        {
+            // Roll back any in-flight batch with this ID.
+            _batchEngine.Clear(); // simplified: in production this only removes the specific batch
+            return new PubAck
+            {
+                ErrorCode = AtomicBatchPublishErrorCodes.InvalidCommit,
+                Stream = stream.Config.Name,
+            };
+        }
+
+        var req = new BatchPublishRequest
+        {
+            BatchId = options.BatchId!,
+            BatchSeq = options.BatchSeq,
+            Subject = subject,
+            Payload = payload,
+            IsCommit = isCommit,
+            CommitValue = commitValue,
+            MsgId = options.MsgId,
+            ExpectedLastSeq = options.ExpectedLastSeq,
+            ExpectedLastSubjectSeq = options.ExpectedLastSubjectSeq,
+            ExpectedLastSubjectSeqSubject = options.ExpectedLastSubjectSeqSubject,
+        };
+
+        var result = _batchEngine.Process(
+            req,
+            _preconditions,
+            stream.Config.DuplicateWindowMs,
+            staged =>
+            {
+                // Check expected last sequence.
+                if (staged.ExpectedLastSeq > 0)
+                {
+                    var st = stream.Store.GetStateAsync(default).GetAwaiter().GetResult();
+                    if (st.LastSeq != staged.ExpectedLastSeq)
+                        return new PubAck { ErrorCode = 10071, Stream = stream.Config.Name };
+                }
+
+                var captured = _streamManager.Capture(staged.Subject, staged.Payload);
+                return captured ?? new PubAck { Stream = stream.Config.Name };
+            });
+
+        return result.Kind switch
+        {
+            AtomicBatchResult.ResultKind.Staged => new PubAck
+            {
+                Stream = stream.Config.Name,
+                // Empty ack for staged (flow control).
+            },
+            AtomicBatchResult.ResultKind.Committed => result.CommitAck!,
+            AtomicBatchResult.ResultKind.Error => new PubAck
+            {
+                ErrorCode = result.ErrorCode,
+                Stream = stream.Config.Name,
+            },
+            _ => new PubAck { Stream = stream.Config.Name },
+        };
+    }
+
+    /// <summary>
+    /// Clears all in-flight batches (called when stream is disabled or deleted).
+    /// Go: server/jetstream_batching.go streamBatches.cleanup()
+    /// </summary>
+    public void ClearBatches() => _batchEngine.Clear();
 }