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/ProgressBarHandler.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 ProgressBar control.
+/// </summary>
+public partial class ProgressBarHandler : ViewHandler<IProgress, SkiaProgressBar>
+{
+    public static IPropertyMapper<IProgress, ProgressBarHandler> Mapper = new PropertyMapper<IProgress, ProgressBarHandler>(ViewHandler.ViewMapper)
+    {
+        [nameof(IProgress.Progress)] = MapProgress,
+        [nameof(IProgress.ProgressColor)] = MapProgressColor,
+        [nameof(IView.IsEnabled)] = MapIsEnabled,
+    };
+
+    public static CommandMapper<IProgress, ProgressBarHandler> CommandMapper = new(ViewHandler.ViewCommandMapper);
+
+    public ProgressBarHandler() : base(Mapper, CommandMapper) { }
+
+    protected override SkiaProgressBar CreatePlatformView() => new SkiaProgressBar();
+
+    public static void MapProgress(ProgressBarHandler handler, IProgress progress)
+    {
+        handler.PlatformView.Progress = progress.Progress;
+    }
+
+    public static void MapProgressColor(ProgressBarHandler handler, IProgress progress)
+    {
+        if (progress.ProgressColor != null)
+            handler.PlatformView.ProgressColor = progress.ProgressColor.ToSKColor();
+        handler.PlatformView.Invalidate();
+    }
+
+    public static void MapIsEnabled(ProgressBarHandler handler, IProgress progress)
+    {
+        handler.PlatformView.IsEnabled = progress.IsEnabled;
+        handler.PlatformView.Invalidate();
+    }
+}