feat(networking): expand gateway reply mapper and add leaf solicited connections (D4+D5)

D4: Add hash segment support to ReplyMapper (_GR_.{cluster}.{hash}.{reply}),
FNV-1a ComputeReplyHash, TryExtractClusterId/Hash, legacy format compat.
D5: Add ConnectSolicitedAsync with exponential backoff (1s-60s cap),
JetStreamDomain propagation in LEAF handshake, LeafNodeOptions.JetStreamDomain.

src/NATS.Server/Configuration/LeafNodeOptions.cs

@@ -5,4 +5,11 @@ public sealed class LeafNodeOptions
     public string Host { get; set; } = "0.0.0.0";
     public int Port { get; set; }
     public List<string> Remotes { get; set; } = [];
+
+    /// <summary>
+    /// JetStream domain for this leaf node. When set, the domain is propagated
+    /// during the leaf handshake for domain-aware JetStream routing.
+    /// Go reference: leafnode.go — JsDomain in leafNodeCfg.
+    /// </summary>
+    public string? JetStreamDomain { get; set; }
 }