feat: add peer management with stream reassignment (Gap 2.4)

Add ProcessAddPeer/ProcessRemovePeer to JetStreamMetaGroup for
peer-driven stream reassignment. Includes AddKnownPeer/RemoveKnownPeer
tracked in a HashSet, RemovePeerFromStream, RemapStreamAssignment with
replacement-peer selection from the known pool, and DesiredReplicas on
RaftGroup for under-replication detection. Go ref: jetstream_cluster.go:2290-2439.

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

@@ -14,6 +14,18 @@ public sealed class RaftGroup
     public string Cluster { get; set; } = string.Empty;
     public string Preferred { get; set; } = string.Empty;
 
+    /// <summary>
+    /// Optional desired replica count for this group.
+    /// When set, ProcessAddPeer uses this to detect under-replication.
+    /// Go reference: jetstream_cluster.go:2284 sa.missingPeers() — len(Peers) &lt; Config.Replicas.
+    /// </summary>
+    public int DesiredReplicas { get; set; }
+
+    /// <summary>
+    /// True when <see cref="DesiredReplicas"/> has been explicitly set (non-zero).
+    /// </summary>
+    public bool HasDesiredReplicas => DesiredReplicas > 0;
+
     public int QuorumSize => (Peers.Count / 2) + 1;
     public bool HasQuorum(int ackCount) => ackCount >= QuorumSize;
 }