build: add InternalsVisibleTo for OpenNest.Tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/OpenNest.Engine.csproj

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