feat: add RaftGroup lifecycle methods (Gap 2.9)

Implement IsMember, SetPreferred, RemovePeer, AddPeer, CreateRaftGroup
factory, IsUnderReplicated, and IsOverReplicated on RaftGroup. Add 22
RaftGroupLifecycleTests covering all helpers and quorum size calculation.

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

@@ -28,6 +28,80 @@ public sealed class RaftGroup
 
     public int QuorumSize => (Peers.Count / 2) + 1;
     public bool HasQuorum(int ackCount) => ackCount >= QuorumSize;
+
+    /// <summary>
+    /// True when the group has fewer peers than the desired replica count.
+    /// Go reference: jetstream_cluster.go missingPeers — len(rg.Peers) &lt; DesiredReplicas.
+    /// </summary>
+    public bool IsUnderReplicated => HasDesiredReplicas && Peers.Count < DesiredReplicas;
+
+    /// <summary>
+    /// True when the group has more peers than the desired replica count.
+    /// Go reference: jetstream_cluster.go extraPeers — len(rg.Peers) &gt; DesiredReplicas.
+    /// </summary>
+    public bool IsOverReplicated => HasDesiredReplicas && Peers.Count > DesiredReplicas;
+
+    /// <summary>
+    /// Returns true if the given peerId is a member of this group (case-sensitive).
+    /// Go reference: jetstream_cluster.go isMember helper.
+    /// </summary>
+    public bool IsMember(string peerId) => Peers.Contains(peerId, StringComparer.Ordinal);
+
+    /// <summary>
+    /// Sets the preferred leader peer for this group.
+    /// Throws <see cref="InvalidOperationException"/> if peerId is not a member.
+    /// Go reference: jetstream_cluster.go setPreferred / rg.Preferred assignment.
+    /// </summary>
+    public void SetPreferred(string peerId)
+    {
+        if (!IsMember(peerId))
+            throw new InvalidOperationException(
+                $"Cannot set preferred peer '{peerId}': not a member of RAFT group '{Name}'.");
+        Preferred = peerId;
+    }
+
+    /// <summary>
+    /// Removes a peer from the group. If the removed peer was the preferred peer,
+    /// <see cref="Preferred"/> is cleared. Returns true if the peer was found and removed.
+    /// Go reference: jetstream_cluster.go removePeer.
+    /// </summary>
+    public bool RemovePeer(string peerId)
+    {
+        var removed = Peers.Remove(peerId);
+        if (removed && string.Equals(Preferred, peerId, StringComparison.Ordinal))
+            Preferred = string.Empty;
+        return removed;
+    }
+
+    /// <summary>
+    /// Adds a peer to the group if not already present. Returns true if the peer was added.
+    /// Go reference: jetstream_cluster.go addPeer / expandGroup.
+    /// </summary>
+    public bool AddPeer(string peerId)
+    {
+        if (IsMember(peerId))
+            return false;
+        Peers.Add(peerId);
+        return true;
+    }
+
+    /// <summary>
+    /// Factory method that uses <see cref="PlacementEngine.SelectPeerGroup"/> to create a
+    /// <see cref="RaftGroup"/> with topology-aware peer selection and sets
+    /// <see cref="DesiredReplicas"/> to the requested replica count.
+    /// Go reference: jetstream_cluster.go createGroupForStream — calls selectPeerGroup then
+    /// assigns rg.DesiredReplicas = replicas.
+    /// </summary>
+    public static RaftGroup CreateRaftGroup(
+        string groupName,
+        int replicas,
+        IReadOnlyList<PeerInfo> availablePeers,
+        PlacementPolicy? policy = null)
+    {
+        var group = PlacementEngine.SelectPeerGroup(groupName, replicas, availablePeers, policy);
+        group.DesiredReplicas = replicas;
+        return group;
+    }
 }
 
 /// <summary>