feat: add stream replica groups and leader stepdown

src/NATS.Server/JetStream/StreamManager.cs

@@ -14,6 +14,8 @@ public sealed class StreamManager
     private readonly JetStreamMetaGroup? _metaGroup;
     private readonly ConcurrentDictionary<string, StreamHandle> _streams =
         new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<string, StreamReplicaGroup> _replicaGroups =
+        new(StringComparer.Ordinal);
     private readonly ConcurrentDictionary<string, List<MirrorCoordinator>> _mirrorsByOrigin =
         new(StringComparer.Ordinal);
     private readonly ConcurrentDictionary<string, List<SourceCoordinator>> _sourcesByOrigin =
@@ -36,6 +38,12 @@ public sealed class StreamManager
             normalized.Name,
             _ => new StreamHandle(normalized, new MemStore()),
             (_, existing) => existing with { Config = normalized });
+        _replicaGroups.AddOrUpdate(
+            normalized.Name,
+            _ => new StreamReplicaGroup(normalized.Name, normalized.Replicas),
+            (_, existing) => existing.Nodes.Count == Math.Max(normalized.Replicas, 1)
+                ? existing
+                : new StreamReplicaGroup(normalized.Name, normalized.Replicas));
         RebuildReplicationCoordinators();
         _metaGroup?.ProposeCreateStreamAsync(normalized, default).GetAwaiter().GetResult();
 
@@ -77,6 +85,9 @@ public sealed class StreamManager
         if (stream == null)
             return null;
 
+        if (_replicaGroups.TryGetValue(stream.Config.Name, out var replicaGroup))
+            _ = replicaGroup.ProposeAsync($"PUB {subject}", default).GetAwaiter().GetResult();
+
         var seq = stream.Store.AppendAsync(subject, payload, default).GetAwaiter().GetResult();
         EnforceLimits(stream);
         var stored = stream.Store.LoadAsync(seq, default).GetAwaiter().GetResult();
@@ -90,6 +101,14 @@ public sealed class StreamManager
         };
     }
 
+    public Task StepDownStreamLeaderAsync(string stream, CancellationToken ct)
+    {
+        if (_replicaGroups.TryGetValue(stream, out var replicaGroup))
+            return replicaGroup.StepDownAsync(ct);
+
+        return Task.CompletedTask;
+    }
+
     private static StreamConfig NormalizeConfig(StreamConfig config)
     {
         var copy = new StreamConfig