fix(io): remove zero-sweep arcs during DXF import

DXF files can contain degenerate arcs where start angle equals end angle
(zero sweep), often left as construction artifacts by CAD software.
These create spurious shapes in ShapeBuilder — e.g. SULLYS-033.dxf
showed 5 loops instead of 4 (3 cutouts + perimeter).

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

OpenNest.IO/CadImporter.cs

@@ -27,6 +27,7 @@ namespace OpenNest.IO
             var dxf = Dxf.Import(path);
 
             RemoveDuplicateArcs(dxf.Entities);
+            RemoveZeroSweepArcs(dxf.Entities);
 
             var bends = new List<Bend>();
             if (options.DetectBends && dxf.Document != null)
@@ -141,6 +142,12 @@ namespace OpenNest.IO
             return drawing;
         }
 
+        internal static void RemoveZeroSweepArcs(List<Entity> entities)
+        {
+            entities.RemoveAll(e =>
+                e is Arc arc && arc.StartAngle.IsEqualTo(arc.EndAngle, Tolerance.ChainTolerance));
+        }
+
         internal static void RemoveDuplicateArcs(List<Entity> entities)
         {
             var circles = entities.OfType<Circle>().ToList();