Move all selection state and operations (SelectedParts, SelectedCutOffs, DeselectAll, SelectAll, AlignSelected, RotateSelectedParts, PushSelected, GetPartAt*, GetPartsFromWindow, DeleteSelected) into a new internal SelectionManager class. PlateView retains public forwarding methods and properties to preserve the existing API surface. SelectedCutOff property kept public for WinForms designer compatibility.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
