Add gateway registration resync and slim HELLO transport frames

Introduce GatewayRegistrationResyncService to recover stale registrations,
extract IGatewayTransportClient interface, add EndpointsUpdate and
RegistrationResyncRequest frame types, and expand transport test coverage.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Router/__Libraries/StellaOps.Router.Common/Models/HeartbeatPayload.cs

@@ -7,6 +7,11 @@ namespace StellaOps.Router.Common.Models;
 /// </summary>
 public sealed record HeartbeatPayload
 {
+    /// <summary>
+    /// Gets the instance descriptor.
+    /// </summary>
+    public InstanceDescriptor? Instance { get; init; }
+
     /// <summary>
     /// Gets the instance ID.
     /// </summary>