Initial commit: .NET MAUI Linux Platform

Complete Linux platform implementation for .NET MAUI with:

- 35+ Skia-rendered controls (Button, Label, Entry, CarouselView, etc.)
- Platform services (Clipboard, FilePicker, Notifications, DragDrop, etc.)
- Accessibility support (AT-SPI2, High Contrast)
- HiDPI and Input Method support
- 216 unit tests
- CI/CD workflows
- Project templates
- Documentation

🤖 Generated with Claude Code

Handlers/ActivityIndicatorHandler.Linux.cs

@@ -0,0 +1,43 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Microsoft.Maui.Handlers;
+
+namespace Microsoft.Maui.Platform;
+
+/// <summary>
+/// Linux handler for ActivityIndicator control.
+/// </summary>
+public partial class ActivityIndicatorHandler : ViewHandler<IActivityIndicator, SkiaActivityIndicator>
+{
+    public static IPropertyMapper<IActivityIndicator, ActivityIndicatorHandler> Mapper = new PropertyMapper<IActivityIndicator, ActivityIndicatorHandler>(ViewHandler.ViewMapper)
+    {
+        [nameof(IActivityIndicator.IsRunning)] = MapIsRunning,
+        [nameof(IActivityIndicator.Color)] = MapColor,
+        [nameof(IView.IsEnabled)] = MapIsEnabled,
+    };
+
+    public static CommandMapper<IActivityIndicator, ActivityIndicatorHandler> CommandMapper = new(ViewHandler.ViewCommandMapper);
+
+    public ActivityIndicatorHandler() : base(Mapper, CommandMapper) { }
+
+    protected override SkiaActivityIndicator CreatePlatformView() => new SkiaActivityIndicator();
+
+    public static void MapIsRunning(ActivityIndicatorHandler handler, IActivityIndicator activityIndicator)
+    {
+        handler.PlatformView.IsRunning = activityIndicator.IsRunning;
+    }
+
+    public static void MapColor(ActivityIndicatorHandler handler, IActivityIndicator activityIndicator)
+    {
+        if (activityIndicator.Color != null)
+            handler.PlatformView.Color = activityIndicator.Color.ToSKColor();
+        handler.PlatformView.Invalidate();
+    }
+
+    public static void MapIsEnabled(ActivityIndicatorHandler handler, IActivityIndicator activityIndicator)
+    {
+        handler.PlatformView.IsEnabled = activityIndicator.IsEnabled;
+        handler.PlatformView.Invalidate();
+    }
+}