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();

OpenNest/Forms/MainForm.cs

@@ -501,6 +501,33 @@ namespace OpenNest.Forms
             activeForm.PlateView.SetAction(typeof(ActionSelectArea));
         }
 
+        private void BestFitViewer_Click(object sender, EventArgs e)
+        {
+            if (activeForm == null)
+                return;
+
+            var plate = activeForm.PlateView.Plate;
+            var drawing = activeForm.Nest.Drawings.Count > 0
+                ? activeForm.Nest.Drawings.First()
+                : null;
+
+            if (drawing == null)
+            {
+                MessageBox.Show("No drawings available.", "Best-Fit Viewer",
+                    MessageBoxButtons.OK, MessageBoxIcon.Information);
+                return;
+            }
+
+            using (var form = new BestFitViewerForm(drawing, plate))
+            {
+                if (form.ShowDialog(this) == DialogResult.OK && form.SelectedResult != null)
+                {
+                    var parts = NestEngine.BuildPairParts(form.SelectedResult, drawing);
+                    activeForm.PlateView.SetAction(typeof(ActionClone), parts);
+                }
+            }
+        }
+
         private void SetOffsetIncrement_Click(object sender, EventArgs e)
         {
             if (activeForm == null) return;
@@ -645,6 +672,7 @@ namespace OpenNest.Forms
                     : activeForm.PlateView.Plate;
 
                 var engine = new NestEngine(plate);
+                engine.CreateEvaluator = GpuEvaluatorFactory.Create;
 
                 if (!engine.Pack(items))
                     break;
@@ -718,6 +746,7 @@ namespace OpenNest.Forms
                 return;
 
             var engine = new NestEngine(activeForm.PlateView.Plate);
+            engine.CreateEvaluator = GpuEvaluatorFactory.Create;
             engine.Fill(new NestItem
             {
                 Drawing = drawing

OpenNest/GpuEvaluatorFactory.cs

@@ -0,0 +1,22 @@
+using System.Diagnostics;
+using OpenNest.Engine.BestFit;
+using OpenNest.Gpu;
+
+namespace OpenNest
+{
+    internal static class GpuEvaluatorFactory
+    {
+        public static IPairEvaluator Create(Drawing drawing, double spacing)
+        {
+            try
+            {
+                return new GpuPairEvaluator(drawing, spacing);
+            }
+            catch
+            {
+                Debug.WriteLine("[GpuEvaluatorFactory] GPU not available, falling back to CPU");
+                return null;
+            }
+        }
+    }
+}

OpenNest/OpenNest.csproj

@@ -13,6 +13,7 @@
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
     <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />
+    <ProjectReference Include="..\OpenNest.Gpu\OpenNest.Gpu.csproj" />
     <PackageReference Include="ACadSharp" Version="3.1.32" />
     <PackageReference Include="System.Drawing.Common" Version="8.0.10" />
   </ItemGroup>