Add background tasks section to WPF + Windows App SDK page

New '## Run code in the background' section with a comparison table of
the three approaches (WAS background tasks, Task Scheduler, Worker Services),
packaging requirements for each, and a note on how Application.Startup
maps to App.OnLaunched for WAS background task registration.

Fixes Gap 10 from ADO #568630.

Co-authored-by: Copilot <[email protected]>

Author: GrantMeStrength

hub/apps/windows-app-sdk/migrate-to-windows-app-sdk/wpf-plus-winappsdk.md

@@ -149,6 +149,21 @@ Build and run again. Click the button, and confirm that a toast notification is
 > [!NOTE]
 > The steps in this section showed you how to create a *packaged app*. An alternative is to create a *packaged app with external location*. For a reminder of all these terms, see [Advantages and disadvantages of packaging your app](/windows/apps/package-and-deploy/).
 
+## Run code in the background
+
+For WPF apps that need to execute code when the app isn't running, there are three approaches depending on your packaging and workload:
+
+| Approach | Packaging required | Best for |
+|---|---|---|
+| [Windows App SDK background tasks](../../windows-app-sdk/applifecycle/background-tasks.md) | Yes (MSIX) | Power-efficient system-managed triggers (time, network, push) |
+| [Task Scheduler](/windows/win32/taskschd/task-scheduler-start-page) | No | Periodic sync, unpackaged apps |
+| [.NET Worker Services](/dotnet/core/extensions/workers) | No | Long-running headless workloads, any deployment model |
+
+For Windows App SDK background tasks, your WPF app registers a COM component using `BackgroundTaskBuilder` just like a WinUI 3 app — the `Application.Startup` event in WPF maps to the role that `App.OnLaunched` plays in WinUI 3. See [Using background tasks in Windows apps](../../windows-app-sdk/applifecycle/background-tasks.md) for the full walkthrough.
+
+> [!NOTE]
+> Windows App SDK background tasks require MSIX packaging. For unpackaged WPF apps, use Task Scheduler or .NET Worker Services instead.
+
 ## Related topics
 
 * [Windows Presentation Foundation (WPF)](/dotnet/desktop/wpf/)