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/Strategies/FillContext.cs

@@ -1,3 +1,4 @@
+using OpenNest.Engine;
 using OpenNest.Engine.Fill;
 using OpenNest.Geometry;
 using System;
@@ -15,6 +16,7 @@ namespace OpenNest.Engine.Strategies
         public CancellationToken Token { get; init; }
         public IProgress<NestProgress> Progress { get; init; }
         public FillPolicy Policy { get; init; }
+        public PartType PartType { get; set; }
 
         public List<Part> CurrentBest { get; set; }
         /// <summary>For progress reporting only; comparisons use Policy.Comparer.</summary>