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/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());
         }
     }