Add 5 verified files from decompiled production code

Changes:
- GtkWebViewHandler.cs - New native WebKit handler
- GtkWebViewProxy.cs - New proxy for WebView positioning
- WebViewHandler.cs - Fixed navigation event handling
- PageHandler.cs - Added MapBackgroundColor
- SkiaView.cs - Made Arrange() virtual

Also adds CLAUDE.md (instructions) and MERGE_TRACKING.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Handlers/GtkWebViewProxy.cs

@@ -0,0 +1,83 @@
+// 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.Platform;
+using SkiaSharp;
+
+namespace Microsoft.Maui.Platform.Linux.Handlers;
+
+/// <summary>
+/// Proxy view that bridges SkiaView layout to GTK WebView positioning.
+/// </summary>
+public class GtkWebViewProxy : SkiaView
+{
+    private readonly GtkWebViewHandler _handler;
+    private readonly GtkWebViewPlatformView _platformView;
+
+    public GtkWebViewPlatformView PlatformView => _platformView;
+    public bool CanGoBack => _platformView.CanGoBack();
+    public bool CanGoForward => _platformView.CanGoForward();
+
+    public GtkWebViewProxy(GtkWebViewHandler handler, GtkWebViewPlatformView platformView)
+    {
+        _handler = handler;
+        _platformView = platformView;
+    }
+
+    public override void Arrange(SKRect bounds)
+    {
+        base.Arrange(bounds);
+        var windowBounds = TransformToWindow(bounds);
+        _handler.RegisterWithHost(windowBounds);
+    }
+
+    private SKRect TransformToWindow(SKRect localBounds)
+    {
+        float x = localBounds.Left;
+        float y = localBounds.Top;
+
+        for (var parent = Parent; parent != null; parent = parent.Parent)
+        {
+            x += parent.Bounds.Left;
+            y += parent.Bounds.Top;
+        }
+
+        return new SKRect(x, y, x + localBounds.Width, y + localBounds.Height);
+    }
+
+    public override void Draw(SKCanvas canvas)
+    {
+        // Draw transparent placeholder - actual WebView is rendered by GTK
+        using var paint = new SKPaint
+        {
+            Color = new SKColor(0, 0, 0, 0),
+            Style = SKPaintStyle.Fill
+        };
+        canvas.DrawRect(Bounds, paint);
+    }
+
+    public void Navigate(string url)
+    {
+        _platformView.Navigate(url);
+    }
+
+    public void LoadHtml(string html, string? baseUrl = null)
+    {
+        _platformView.LoadHtml(html, baseUrl);
+    }
+
+    public void GoBack()
+    {
+        _platformView.GoBack();
+    }
+
+    public void GoForward()
+    {
+        _platformView.GoForward();
+    }
+
+    public void Reload()
+    {
+        _platformView.Reload();
+    }
+}