feat: implement jetstream governance runtime parity semantics

src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs

@@ -7,6 +7,8 @@ public sealed class JetStreamMetaGroup
 {
     private readonly int _nodes;
     private readonly ConcurrentDictionary<string, byte> _streams = new(StringComparer.Ordinal);
+    private int _leaderIndex = 1;
+    private long _leadershipVersion = 1;
 
     public JetStreamMetaGroup(int nodes)
     {
@@ -25,13 +27,18 @@ public sealed class JetStreamMetaGroup
         {
             Streams = _streams.Keys.OrderBy(x => x, StringComparer.Ordinal).ToArray(),
             ClusterSize = _nodes,
+            LeaderId = $"meta-{_leaderIndex}",
+            LeadershipVersion = _leadershipVersion,
         };
     }
 
     public void StepDown()
     {
-        // Placeholder for parity API behavior; current in-memory meta group
-        // does not track explicit leader state.
+        _leaderIndex++;
+        if (_leaderIndex > Math.Max(_nodes, 1))
+            _leaderIndex = 1;
+
+        Interlocked.Increment(ref _leadershipVersion);
     }
 }
 
@@ -39,4 +46,6 @@ public sealed class MetaGroupState
 {
     public IReadOnlyList<string> Streams { get; init; } = [];
     public int ClusterSize { get; init; }
+    public string LeaderId { get; init; } = string.Empty;
+    public long LeadershipVersion { get; init; }
 }