feat: add PairCandidate, BestFitResult, and IBestFitStrategy for best-fit pair finding

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

OpenNest.Engine/BestFit/IBestFitStrategy.cs

@@ -0,0 +1,11 @@
+using System.Collections.Generic;
+
+namespace OpenNest.Engine.BestFit
+{
+    public interface IBestFitStrategy
+    {
+        int Type { get; }
+        string Description { get; }
+        List<PairCandidate> GenerateCandidates(Drawing drawing, double spacing, double stepSize);
+    }
+}