feat: integrate GPU slide computation into best-fit pipeline

Thread ISlideComputer through BestFitCache → BestFitFinder →
RotationSlideStrategy. RotationSlideStrategy now collects all offsets
across 4 push directions and dispatches them in a single batch (GPU or
CPU fallback). Also improves rotation angle extraction: uses raw geometry
(line endpoints + arc cardinal extremes) instead of tessellation to avoid
flooding the hull with near-duplicate edge angles, and adds a 5-degree
deduplication threshold.

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

OpenNest/Forms/MainForm.cs

@@ -50,6 +50,9 @@ namespace OpenNest.Forms
 
             //if (GpuEvaluatorFactory.GpuAvailable)
             //    BestFitCache.CreateEvaluator = (drawing, spacing) => GpuEvaluatorFactory.Create(drawing, spacing);
+
+            if (GpuEvaluatorFactory.GpuAvailable)
+                BestFitCache.CreateSlideComputer = () => GpuEvaluatorFactory.CreateSlideComputer();
         }
 
         private Nest CreateDefaultNest()