feat: wire GpuPairEvaluator into NestEngine with auto-detection

NestEngine.CreateEvaluator factory delegate allows injection of GPU
evaluator from UI layer. GpuEvaluatorFactory.Create attempts GPU,
returns null (CPU fallback) if unavailable. All NestEngine call sites
wired up.

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

OpenNest/Actions/ActionClone.cs

@@ -172,6 +172,7 @@ namespace OpenNest.Actions
         {
             var plate = plateView.Plate;
             var engine = new NestEngine(plate);
+            engine.CreateEvaluator = GpuEvaluatorFactory.Create;
             var groupParts = parts.Select(p => p.BasePart).ToList();
 
             var bounds = plate.WorkArea();

OpenNest/Actions/ActionFillArea.cs

@@ -25,10 +25,8 @@ namespace OpenNest.Actions
         private void FillArea()
         {
             var engine = new NestEngine(plateView.Plate);
-            engine.FillArea(SelectedArea, new NestItem
-            {
-                Drawing = drawing
-            });
+            engine.CreateEvaluator = GpuEvaluatorFactory.Create;
+            engine.Fill(new NestItem { Drawing = drawing }, SelectedArea);
 
             plateView.Invalidate();
             Update();