Add OpenNest.Desktop (net8.0), the toolkit-neutral layer the WinForms app
and OpenNest.Avalonia will share, and OpenNest.Desktop.Tests with a guard
that the library references no WinForms/WindowsDesktop, System.Drawing.Common
or Avalonia assembly, project setting or (transitive) package.
Both desktop front ends reference it; OpenNest.Avalonia joins OpenNest.sln
and the new tests join the Linux CI matrix. No code moves yet.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
