feat: add CollisionResult data class for polygon collision detection

Immutable result type that holds overlap flag, overlap regions (as polygons),
intersection points, and computed overlap area.

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

OpenNest.Core/Geometry/CollisionResult.cs

@@ -0,0 +1,23 @@
+using System.Collections.Generic;
+using System.Linq;
+
+namespace OpenNest.Geometry
+{
+    public class CollisionResult
+    {
+        public static readonly CollisionResult None = new(false, new List<Polygon>(), new List<Vector>());
+
+        public CollisionResult(bool overlaps, List<Polygon> overlapRegions, List<Vector> intersectionPoints)
+        {
+            Overlaps = overlaps;
+            OverlapRegions = overlapRegions;
+            IntersectionPoints = intersectionPoints;
+            OverlapArea = overlapRegions.Sum(r => r.Area());
+        }
+
+        public bool Overlaps { get; }
+        public List<Polygon> OverlapRegions { get; }
+        public List<Vector> IntersectionPoints { get; }
+        public double OverlapArea { get; }
+    }
+}