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/Geometry/EllipseConverter.cs

@@ -173,7 +173,11 @@ namespace OpenNest.Geometry
 
             if (maxDev <= tolerance)
             {
-                results.Add(CreateArc(arcCenter, radius, center, semiMajor, semiMinor, rotation, t0, t1));
+                var arc = CreateArc(arcCenter, radius, center, semiMajor, semiMinor, rotation, t0, t1);
+                if (arc.SweepAngle() < Tolerance.Epsilon)
+                    results.Add(new Line(p0, p1));
+                else
+                    results.Add(arc);
             }
             else
             {