fix(io): handle flipped OCS normal on DXF ellipse import

Ellipses with extrusion direction Z=-1 had their parametric direction
reversed, causing the curve to appear mirrored. Negate start/end
parameters when Normal.Z < 0 to correct the minor-axis traversal.

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

OpenNest.IO/OpenNest.IO.csproj

@@ -4,6 +4,9 @@
     <RootNamespace>OpenNest.IO</RootNamespace>
     <AssemblyName>OpenNest.IO</AssemblyName>
   </PropertyGroup>
+  <ItemGroup>
+    <InternalsVisibleTo Include="OpenNest.Tests" />
+  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\OpenNest.Core\OpenNest.Core.csproj" />
     <ProjectReference Include="..\OpenNest.Engine\OpenNest.Engine.csproj" />