revert: switch BestFitFinder back to RotationSlideStrategy

NFP strategy has coordinate correction issues causing overlaps.
The slide-based approach is fast and accurate — keeping it as default.
NfpSlideStrategy and PolygonHelper remain in the codebase for future use.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/BestFit/BestFitFinder.cs

@@ -83,8 +83,8 @@ namespace OpenNest.Engine.BestFit
 
             foreach (var angle in angles)
             {
-                var desc = $"{Angle.ToDegrees(angle):F1} deg NFP";
+                var desc = string.Format("{0:F1} deg rotated, offset slide", Angle.ToDegrees(angle));
-                strategies.Add(new NfpSlideStrategy(angle, type++, desc));
+                strategies.Add(new RotationSlideStrategy(angle, type++, desc, _slideComputer));
             }
 
             return strategies;