fix: remove self-intersecting loops from polygon offset

Polygon offset at concave corners creates geometry that folds back
through itself. Added RemoveSelfIntersections() to Polygon that
detects non-adjacent edge crossings and removes the smaller loop
at each crossing. Applied to both collision detection and rendering.

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

OpenNest.Core/Helper.cs

@@ -789,6 +789,7 @@ namespace OpenNest
                     continue;
 
                 var polygon = offsetEntity.ToPolygonWithTolerance(PushChordTolerance);
+                polygon.RemoveSelfIntersections();
                 polygon.Offset(part.Location);
                 lines.AddRange(polygon.ToLines());
             }
@@ -810,6 +811,7 @@ namespace OpenNest
                     continue;
 
                 var polygon = offsetEntity.ToPolygonWithTolerance(PushChordTolerance);
+                polygon.RemoveSelfIntersections();
                 polygon.Offset(part.Location);
                 lines.AddRange(GetDirectionalLines(polygon, facingDirection));
             }