fix(core): emit line instead of arc for near-zero sweep to avoid full-circle misinterpretation

Near-zero-sweep arcs with large radius (e.g. from ellipse converter) have
nearly-coincident start/end points. Downstream code (ConvertProgram, Program
BoundingBox) treats coincident start/end as a full 360° circle, inflating the
bounding box and rendering wrong geometry. Emit a LinearMove when sweep is
negligible — geometrically equivalent and avoids the ambiguity. Also fix the
ellipse converter to produce lines instead of degenerate arcs at the source.

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

OpenNest.Core/Converters/ConvertGeometry.cs

@@ -1,5 +1,6 @@
 using OpenNest.CNC;
 using OpenNest.Geometry;
+using OpenNest.Math;
 using System.Collections.Generic;
 
 namespace OpenNest.Converters
@@ -86,7 +87,16 @@ namespace OpenNest.Converters
 
             lastpt = endpt;
 
-            pgm.ArcTo(endpt, arc.Center, arc.IsReversed ? RotationType.CW : RotationType.CCW);
+            var sweep = System.Math.Abs(arc.SweepAngle());
+            if (sweep < Tolerance.Epsilon || sweep.IsEqualTo(Angle.TwoPI))
+            {
+                pgm.LineTo(endpt);
+            }
+            else
+            {
+                pgm.ArcTo(endpt, arc.Center, arc.IsReversed ? RotationType.CW : RotationType.CCW);
+            }
+
             return lastpt;
         }