feat: add ExpressionEvaluator for G-code variable expressions

Also set ContinueOnError=true on Cincinnati's post-build copy to prevent
the running WinForms app from blocking test builds via a file lock.

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

OpenNest.Posts.Cincinnati/OpenNest.Posts.Cincinnati.csproj

@@ -11,6 +11,6 @@
       <PostsDir>..\OpenNest\bin\$(Configuration)\$(TargetFramework)\Posts\</PostsDir>
     </PropertyGroup>
     <MakeDir Directories="$(PostsDir)" />
-    <Copy SourceFiles="$(TargetPath)" DestinationFolder="$(PostsDir)" SkipUnchangedFiles="true" />
+    <Copy SourceFiles="$(TargetPath)" DestinationFolder="$(PostsDir)" SkipUnchangedFiles="true" ContinueOnError="true" />
   </Target>
 </Project>