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
+        _leaderIndex++;
-        // does not track explicit leader state.
+        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; }
 }

src/NATS.Server/JetStream/StreamManager.cs

@@ -34,6 +34,7 @@ public sealed class StreamManager
     }
 
     public IReadOnlyCollection<string> StreamNames => _streams.Keys.ToArray();
+    public MetaGroupState? GetMetaState() => _metaGroup?.GetState();
 
     public IReadOnlyList<string> ListNames()
         => [.. _streams.Keys.OrderBy(x => x, StringComparer.Ordinal)];

tests/NATS.Server.Tests/JetStream/JetStreamMetaGovernanceStrictParityTests.cs

@@ -0,0 +1,25 @@
+using NATS.Server.JetStream.Cluster;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamMetaGovernanceStrictParityTests
+{
+    [Fact]
+    public async Task Meta_and_replica_governance_actions_reflect_committed_state_transitions()
+    {
+        var meta = new JetStreamMetaGroup(3);
+        var before = meta.GetState();
+
+        await meta.ProposeCreateStreamAsync(new NATS.Server.JetStream.Models.StreamConfig
+        {
+            Name = "ORDERS_GOV",
+            Subjects = ["orders.gov"],
+        }, default);
+        meta.StepDown();
+        var after = meta.GetState();
+
+        after.Streams.ShouldContain("ORDERS_GOV");
+        after.LeaderId.ShouldNotBe(before.LeaderId);
+        after.LeadershipVersion.ShouldBe(before.LeadershipVersion + 1);
+    }
+}

tests/NATS.Server.Tests/JetStream/JetStreamReplicaGovernanceStrictParityTests.cs

@@ -0,0 +1,19 @@
+using NATS.Server.JetStream.Cluster;
+
+namespace NATS.Server.Tests.JetStream;
+
+public class JetStreamReplicaGovernanceStrictParityTests
+{
+    [Fact]
+    public async Task Meta_and_replica_governance_actions_reflect_committed_state_transitions()
+    {
+        var group = new StreamReplicaGroup("ORDERS_GOV", replicas: 3);
+        var beforeLeader = group.Leader.Id;
+
+        await group.StepDownAsync(default);
+        group.Leader.Id.ShouldNotBe(beforeLeader);
+
+        var index = await group.ProposeAsync("set placement", default);
+        index.ShouldBeGreaterThan(0);
+    }
+}