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>

Views/SkiaView.cs

@@ -926,7 +926,7 @@ public abstract class SkiaView : BindableObject, IDisposable
     /// <summary>
     /// Arranges this view within the given bounds.
     /// </summary>
-    public void Arrange(SKRect bounds)
+    public virtual void Arrange(SKRect bounds)
     {
         Bounds = ArrangeOverride(bounds);
     }