feat(popout): wire pool + window render + sidebar pop-out routing

HellionChat/Infrastructure/Hosting/InitHostedServices.cs

@@ -164,3 +164,23 @@ internal sealed class CommandHelpWindowInitHostedService(
 
     public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 }
+
+// Attaches the singleton PayloadHandler to every pre-allocated pop-out
+// window's MessageList post-container-build. Pool/window cannot take the
+// PayloadHandler via ctor (that would close the silent FactoryCallSite cycle —
+// same §6.2 reason as MessageList.AttachPayloadHandler / CommandHelpWindow.
+// AttachMainWindow). Both singletons exist by host.StartAsync time.
+internal sealed class ChannelPopoutInitHostedService(
+    ChannelPopoutPool pool,
+    PayloadHandler payloadHandler
+) : IHostedService
+{
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        foreach (var window in pool.Instances)
+            window.AttachPayloadHandler(payloadHandler);
+        return Task.CompletedTask;
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}