refactor: split large files into partial classes by concern

Split LinuxApplication into Input and Lifecycle partials. Extract SkiaView into Accessibility, Drawing, and Input partials. Split SkiaEntry and SkiaEditor into Drawing and Input partials. Extract TextRenderingHelper from SkiaRenderingEngine. Create dedicated files for SkiaAbsoluteLayout, SkiaGrid, and SkiaStackLayout. This reduces file sizes from 40K+ lines to manageable units organized by responsibility.

Rendering/SkiaRenderingEngine.cs

@@ -26,8 +26,15 @@ public class SkiaRenderingEngine : IDisposable
     // Dirty region tracking for optimized rendering
     private readonly List<SKRect> _dirtyRegions = new();
     private readonly object _dirtyLock = new();
-    private const int MaxDirtyRegions = 32;
-    private const float RegionMergeThreshold = 0.3f; // Merge if overlap > 30%
+    /// <summary>
+    /// Maximum number of dirty regions to track before falling back to a full redraw.
+    /// </summary>
+    public static int MaxDirtyRegions { get; set; } = 32;
+
+    /// <summary>
+    /// Overlap ratio threshold (0.0-1.0) at which adjacent dirty regions are merged.
+    /// </summary>
+    public static float RegionMergeThreshold { get; set; } = 0.3f;
 
     public static SkiaRenderingEngine? Current { get; private set; }
     public ResourceCache ResourceCache { get; }