Document and Undo/ (ChangeScope, NestFingerprint, NestUndoState) use
only Core, Data, IO and Engine, so they move into the toolkit-neutral
library both desktop front ends share. Namespaces are unchanged, so the
WinForms app compiles as before; it keeps using the internal scope
factories and fingerprint through InternalsVisibleTo.
The scope, fingerprint and SaveAs/save-point rules now have Linux tests
in OpenNest.Desktop.Tests; before, only the Windows-only WinForms suite
covered them.