Add WinUI main thread scheduler support for MAUI

Introduces WinUIMauiMainThreadScheduler for WinUI/.NET MAUI main UI thread scheduling, guarded by WINUI_TARGET. Updates platform preprocessor directives to include TVOS and ensures correct scheduler is used per platform in WithMauiScheduler.

Author: ChrisPulman

src/ReactiveUI.Maui/Builder/MauiReactiveUIBuilderExtensions.cs

@@ -20,6 +20,16 @@ public static class MauiReactiveUIBuilderExtensions
     /// </value>
     public static IScheduler MauiMainThreadScheduler { get; } = DefaultScheduler.Instance;
 
+#if WINUI_TARGET
+    /// <summary>
+    /// Gets a scheduler that schedules work on the WinUI or .NET MAUI main UI thread, if available.
+    /// </summary>
+    /// <remarks>Use this scheduler to ensure that actions are executed on the main thread in WinUI or .NET
+    /// MAUI applications. This is useful for updating UI elements or performing operations that require main thread
+    /// access. If called from a non-main thread, scheduled actions will be marshaled to the main UI thread.</remarks>
+    public static IScheduler WinUIMauiMainThreadScheduler { get; } = new WaitForDispatcherScheduler(static () => DispatcherQueueScheduler.Current);
+#endif
+
 #if ANDROID
     /// <summary>
     /// Gets the scheduler that schedules work on the Android main (UI) thread.
@@ -30,7 +40,7 @@ public static class MauiReactiveUIBuilderExtensions
     public static IScheduler AndroidMainThreadScheduler { get; } = HandlerScheduler.MainThreadScheduler;
 #endif
 
-#if MACCATALYST || IOS || MACOS
+#if MACCATALYST || IOS || MACOS || TVOS
     /// <summary>
     /// Gets the scheduler that schedules work on the Apple main (UI) thread.
     /// </summary>
@@ -58,9 +68,7 @@ public static IReactiveUIBuilder WithMaui(this IReactiveUIBuilder builder, IDisp
         }
 
         return builder
-#if !WINUI_TARGET
             .WithMauiScheduler()
-#endif
             .WithPlatformModule<Maui.Registrations>();
     }
 
@@ -76,10 +84,14 @@ public static IReactiveUIBuilder WithMauiScheduler(this IReactiveUIBuilder build
             throw new ArgumentNullException(nameof(builder));
         }
 
+        builder.WithTaskPoolScheduler(TaskPoolScheduler.Default);
+
 #if ANDROID
         return builder.WithMainThreadScheduler(AndroidMainThreadScheduler);
-#elif MACCATALYST || IOS || MACOS
+#elif MACCATALYST || IOS || MACOS || TVOS
         return builder.WithMainThreadScheduler(AppleMainThreadScheduler);
+#elif WINUI_TARGET
+        return builder.WithMainThreadScheduler(WinUIMauiMainThreadScheduler);
 #else
         return builder.WithMainThreadScheduler(MauiMainThreadScheduler);
 #endif