ObservableList.Remove fired ItemRemoved even when the item wasn't in
the list, causing Plate to decrement Quantity.Nested for clone preview
parts that were never added — producing -1 counts. Delete in PlateView
now cancels ActionClone instead of trying to remove its preview parts.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>