fix: convert arc angles from radians to degrees in DrawingDxfExporter

ACadSharp Arc.StartAngle/EndAngle store values in degrees (DXF spec groups
50/51). The previous code passed Math.Atan2 results (radians) directly,
causing silent geometry corruption. Also hardened the arc export test to
assert angles are in degree range (0..360).

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

PepLib.Core.Tests/IO/DrawingDxfExporterTests.cs

@@ -59,5 +59,9 @@ public class DrawingDxfExporterTests
         var arcs = doc.Entities.OfType<ACadSharp.Entities.Arc>().ToList();
         Assert.Single(lines);
         Assert.Single(arcs);
+        var arc = doc.Entities.OfType<ACadSharp.Entities.Arc>().Single();
+        // Angles must be in degrees (0..360), not radians (0..6.28)
+        Assert.InRange(arc.StartAngle, 0, 360);
+        Assert.InRange(arc.EndAngle, 0, 360);
     }
 }