feat: execute post-baseline jetstream parity plan

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

@@ -42,6 +42,30 @@ public sealed class StreamReplicaGroup
         return Task.CompletedTask;
     }
 
+    public Task ApplyPlacementAsync(IReadOnlyList<int> placement, CancellationToken ct)
+    {
+        _ = ct;
+        var targetCount = Math.Max(placement.Count, 1);
+        if (targetCount == _nodes.Count)
+            return Task.CompletedTask;
+
+        if (targetCount > _nodes.Count)
+        {
+            for (var i = _nodes.Count + 1; i <= targetCount; i++)
+                _nodes.Add(new RaftNode($"{streamNamePrefix()}-r{i}"));
+        }
+        else
+        {
+            _nodes.RemoveRange(targetCount, _nodes.Count - targetCount);
+        }
+
+        foreach (var node in _nodes)
+            node.ConfigureCluster(_nodes);
+
+        Leader = ElectLeader(_nodes[0]);
+        return Task.CompletedTask;
+    }
+
     private RaftNode SelectNextCandidate(RaftNode currentLeader)
     {
         if (_nodes.Count == 1)
@@ -62,4 +86,6 @@ public sealed class StreamReplicaGroup
 
         return candidate;
     }
+
+    private string streamNamePrefix() => StreamName.ToLowerInvariant();
 }