Covers setup, running, debugging, publishing to the Microsoft Store, App_Resources/Windows, the windows config block, marshalling, native code, subclassing and the WinUI 3 native component of each UI element.

- Extending WinRT and .NET classes: class syntax, extend(), super, constructor
  arguments, interfaces, @NativeClass/ES5 output, lifetime; no build step.
- C# sources in App_Resources/Windows and plugin platforms/windows are compiled
  into the app and reachable by namespace; NuGet namespaces need no
  registration; Tasks are awaitable; JS functions and plain objects for
  delegates and structs.
- Background-thread callbacks run on the UI thread; the layout-mutation crash
  warning is gone; static events on class constructors.