feat: add reverse push directions for concave interlocking and cache best-fit results

Add PushDirection.Right and PushDirection.Up to RotationSlideStrategy so
parts can approach from all four directions. This discovers concave
interlocking arrangements (e.g. L-shaped parts nesting into each other's
cavities) that the original Left/Down-only slides could never reach.

Introduce BestFitCache so best-fit results are computed once at step size
0.25 and shared between the viewer and nesting engine. The GPU evaluator
factory is configured once at startup instead of being wired per call
site, and NestEngine.CreateEvaluator is removed.

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

OpenNest/Actions/ActionClone.cs

@@ -4,7 +4,6 @@ using System.Linq;
 using System.Windows.Forms;
 using OpenNest.Controls;
 using OpenNest.Geometry;
-using OpenNest.Gpu;
 
 namespace OpenNest.Actions
 {
@@ -173,7 +172,6 @@ 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

@@ -1,7 +1,6 @@
 using System.ComponentModel;
 using System.Windows.Forms;
 using OpenNest.Controls;
-using OpenNest.Gpu;
 
 namespace OpenNest.Actions
 {
@@ -26,7 +25,6 @@ namespace OpenNest.Actions
         private void FillArea()
         {
             var engine = new NestEngine(plateView.Plate);
-            engine.CreateEvaluator = GpuEvaluatorFactory.Create;
             engine.Fill(new NestItem { Drawing = drawing }, SelectedArea);
 
             plateView.Invalidate();

OpenNest/Forms/BestFitViewerForm.cs

@@ -11,7 +11,6 @@ namespace OpenNest.Forms
         private const int Columns = 5;
         private const int RowHeight = 300;
         private const int MaxResults = 50;
-        private const double ViewerStepSize = 1.0;
 
         private static readonly Color KeptColor = Color.FromArgb(0, 0, 100);
         private static readonly Color DroppedColor = Color.FromArgb(100, 0, 0);
@@ -56,8 +55,8 @@ namespace OpenNest.Forms
         {
             var sw = Stopwatch.StartNew();
 
-            var finder = new BestFitFinder(plate.Size.Width, plate.Size.Height);
-            var results = finder.FindBestFits(drawing, plate.PartSpacing, ViewerStepSize);
+            var results = BestFitCache.GetOrCompute(
+                drawing, plate.Size.Width, plate.Size.Height, plate.PartSpacing);
 
             var findMs = sw.ElapsedMilliseconds;
             var total = results.Count;

OpenNest/Forms/MainForm.cs

@@ -688,7 +688,6 @@ namespace OpenNest.Forms
                     : activeForm.PlateView.Plate;
 
                 var engine = new NestEngine(plate);
-                engine.CreateEvaluator = GpuEvaluatorFactory.Create;
 
                 if (!engine.Pack(items))
                     break;
@@ -762,7 +761,6 @@ namespace OpenNest.Forms
                 return;
 
             var engine = new NestEngine(activeForm.PlateView.Plate);
-            engine.CreateEvaluator = GpuEvaluatorFactory.Create;
             engine.Fill(new NestItem
             {
                 Drawing = drawing

OpenNest/MainApp.cs

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