fix: use CPU evaluator for best-fit cache, remove broken GPU dilation

The GPU bitmap evaluator produces false overlap detections due to
discretization errors at cell boundaries. Use the CPU PairEvaluator
(exact geometric intersection) for now. Also remove the double-counted
spacing dilation from GpuPairEvaluator for when GPU is revisited.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

OpenNest/MainApp.cs

@@ -1,8 +1,6 @@
 using System;
 using System.Windows.Forms;
-using OpenNest.Engine.BestFit;
 using OpenNest.Forms;
-using OpenNest.Gpu;
 
 namespace OpenNest
 {
@@ -13,7 +11,6 @@ namespace OpenNest
         {
             Application.EnableVisualStyles();
             Application.SetCompatibleTextRenderingDefault(false);
-            BestFitCache.CreateEvaluator = GpuEvaluatorFactory.Create;
             Application.Run(new MainForm());
         }
     }