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/Models/StreamConfig.cs

@@ -52,6 +52,36 @@ public sealed class StreamConfig
     // Incompatible with Mirror and Sources.
     // Go reference: server/stream.go:369 (AllowMsgSchedules field)
     public bool AllowMsgSchedules { get; set; }
+
+    // Go: StreamConfig.AllowMsgCounter — enables CRDT counter semantics on messages.
+    // Added in v2.12, requires API level 2.
+    // Go reference: server/stream.go:365 (AllowMsgCounter field)
+    public bool AllowMsgCounter { get; set; }
+
+    // Go: StreamConfig.AllowAtomicPublish — enables atomic batch publishing.
+    // Added in v2.12, requires API level 2.
+    // Go reference: server/stream.go:367 (AllowAtomicPublish field)
+    public bool AllowAtomicPublish { get; set; }
+
+    // Go: StreamConfig.PersistMode — async vs sync storage persistence.
+    // AsyncPersistMode requires API level 2.
+    // Go reference: server/stream.go:375 (PersistMode field)
+    public PersistMode PersistMode { get; set; } = PersistMode.Sync;
+
+    // Go: StreamConfig.Metadata — user-supplied and server-managed key/value metadata.
+    // The server automatically sets _nats.req.level, _nats.ver, _nats.level.
+    // Go reference: server/stream.go:380 (Metadata field)
+    public Dictionary<string, string>? Metadata { get; set; }
+}
+
+/// <summary>
+/// Persistence mode for the stream.
+/// Go reference: server/stream.go — AsyncPersistMode constant.
+/// </summary>
+public enum PersistMode
+{
+    Sync = 0,
+    Async = 1,
 }
 
 public enum StorageType