perf: optimize best fit computation and plate optimizer

- Try all valid best fit pairs instead of only the first when qty=2,
  picking the best via IsBetterFill comparer (fixes suboptimal plate
  selection during auto-nesting)
- Pre-compute best fits across all plate sizes once via
  BestFitCache.ComputeForSizes instead of per-size GPU evaluation
- Early exit plate optimizer when all items fit (salvage < 100%)
- Trim slide offset sweep range to 50% overlap to reduce candidates
- Use actual geometry (ray-arc/ray-circle intersection) instead of
  tessellated polygons for slide distance computation — eliminates
  the massive line count from circle/arc tessellation
- Add RayArcDistance and RayCircleDistance to SpatialQuery
- Add PartGeometry.GetOffsetPerimeterEntities for non-tessellated
  perimeter extraction
- Disable GPU slide computer (slower than CPU currently)
- Remove dead SelectBestFitPair virtual method and overrides

Reduces best fit computation from 7+ minutes to ~4 seconds for a
73x25" part with 30+ holes on a 48x96 plate.

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

OpenNest/Forms/MainForm.cs

@@ -64,8 +64,8 @@ namespace OpenNest.Forms
             //if (GpuEvaluatorFactory.GpuAvailable)
             //    BestFitCache.CreateEvaluator = (drawing, spacing) => GpuEvaluatorFactory.Create(drawing, spacing);
 
-            if (GpuEvaluatorFactory.GpuAvailable)
-                BestFitCache.CreateSlideComputer = () => GpuEvaluatorFactory.CreateSlideComputer();
+            //if (GpuEvaluatorFactory.GpuAvailable)
+            //    BestFitCache.CreateSlideComputer = () => GpuEvaluatorFactory.CreateSlideComputer();
 
             var enginesDir = Path.Combine(Application.StartupPath, "Engines");
             NestEngineRegistry.LoadPlugins(enginesDir);