feat: add JetStream config change reload (Gap 14.6)

Add ApplyJetStreamConfigChanges to ConfigReloader for detecting MaxMemoryStore,
MaxFileStore, and Domain changes during hot reload. Add Domain property to
JetStreamOptions. Port 10 unit tests covering all change permutations.

src/NATS.Server/Configuration/JetStreamOptions.cs

@@ -34,4 +34,11 @@ public sealed class JetStreamOptions
     /// Maps to Go's JetStreamAccountLimits.MaxConsumers (jetstream.go).
     /// </summary>
     public int MaxConsumers { get; set; }
+
+    /// <summary>
+    /// JetStream domain name for this server. Used for domain-scoped routing and
+    /// subject prefix isolation in clustered deployments.
+    /// Maps to Go's Options.JetStreamDomain (opts.go).
+    /// </summary>
+    public string? Domain { get; set; }
 }