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);
     }
 }

PepLib.Core/IO/DrawingDxfExporter.cs

@@ -1,4 +1,4 @@
-using ACadSharp;
+using ACadSharp;
 using ACadSharp.IO;
 using ACadSharp.Tables;
 using CSMath;
@@ -77,8 +77,8 @@ public static class DrawingDxfExporter
                             {
                                 Center = ToXYZ(arcCenter),
                                 Radius = radius,
-                                StartAngle = startAngle,
-                                EndAngle = endAngle,
+                                StartAngle = startAngle * (180.0 / Math.PI),
+                                EndAngle = endAngle * (180.0 / Math.PI),
                                 Layer = layer
                             });
                         }
@@ -104,3 +104,4 @@ public static class DrawingDxfExporter
 
     private static XYZ ToXYZ(Vector v) => new XYZ(v.X, v.Y, 0);
 }
+