feat: wire PartClassifier into engine and update angle selection

Replace RotationAnalysis.FindBestRotation with PartClassifier.Classify in
RunPipeline, propagate ClassificationResult through BuildAngles signatures and
FillContext.PartType, and rewrite AngleCandidateBuilder to dispatch on part type
(Circle=1 angle, Rectangle=2, Irregular=full sweep).

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

OpenNest.Engine/PartClassifier.cs

@@ -25,7 +25,7 @@ namespace OpenNest.Engine
 
         public static ClassificationResult Classify(Drawing drawing)
         {
-            var result = new ClassificationResult();
+            var result = new ClassificationResult { Type = PartType.Irregular };
 
             var entities = ConvertProgram.ToGeometry(drawing.Program)
                 .Where(e => e.Layer != SpecialLayers.Rapid);