feat: add Collision static class with Sutherland-Hodgman clipping and tests

Polygon-polygon collision detection using convex decomposition (ear-clipping
triangulation) followed by Sutherland-Hodgman clipping on each triangle pair.
Handles overlapping, non-overlapping, edge-touching, containment, and concave
polygons. Includes hole subtraction support for future use.

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

OpenNest.Core/Geometry/CollisionResult.cs

@@ -16,8 +16,8 @@ namespace OpenNest.Geometry
         }
 
         public bool Overlaps { get; }
-        public List<Polygon> OverlapRegions { get; }
-        public List<Vector> IntersectionPoints { get; }
+        public IReadOnlyList<Polygon> OverlapRegions { get; }
+        public IReadOnlyList<Vector> IntersectionPoints { get; }
         public double OverlapArea { get; }
     }
 }