The engine selection and the whole-job progress adapter use only Engine
and Data, so they move into the toolkit-neutral library for the Avalonia
front end to share. The namespace stays OpenNest.Forms, so WinForms
callers are unchanged.
Linux tests now cover the adapter's stage routing, one-based plate
numbers and descriptions, and the fill-strategy mapping; the saved
selection is left out because it reads and writes the real per-user file.
