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>