feat(jetstream): add API leader forwarding and stream purge options (C7+C8)

C7: JetStreamApiRouter now checks leadership before mutating operations.
Non-leader nodes return error code 10003 with a leader_hint field.
JetStreamMetaGroup gains IsLeader() and Leader for cluster-aware routing.

C8: StreamApiHandlers.HandlePurge accepts PurgeRequest options (filter,
seq, keep). StreamManager.PurgeEx implements subject-filtered purge,
sequence-based purge, keep-last-N, and filter+keep combinations.

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

@@ -6,15 +6,34 @@ namespace NATS.Server.JetStream.Cluster;
 public sealed class JetStreamMetaGroup
 {
     private readonly int _nodes;
+    private readonly int _selfIndex;
     private readonly ConcurrentDictionary<string, byte> _streams = new(StringComparer.Ordinal);
     private int _leaderIndex = 1;
     private long _leadershipVersion = 1;
 
     public JetStreamMetaGroup(int nodes)
+        : this(nodes, selfIndex: 1)
+    {
+    }
+
+    public JetStreamMetaGroup(int nodes, int selfIndex)
     {
         _nodes = nodes;
+        _selfIndex = selfIndex;
     }
 
+    /// <summary>
+    /// Returns true when this node is the current meta-group leader.
+    /// Go reference: jetstream_api.go:200-300 — leader check before mutating operations.
+    /// </summary>
+    public bool IsLeader() => _leaderIndex == _selfIndex;
+
+    /// <summary>
+    /// Returns the leader identifier string, e.g. "meta-1".
+    /// Used to populate the leader_hint field in not-leader error responses.
+    /// </summary>
+    public string Leader => $"meta-{_leaderIndex}";
+
     public Task ProposeCreateStreamAsync(StreamConfig config, CancellationToken ct)
     {
         _streams[config.Name] = 0;