isPendingDeletion method
Whether key is pending deletion — present in the structural maps but
animating out and scheduled for purge once the animation settles.
Drop-target resolution should skip pending-deletion rows: they are visually vanishing and cannot be valid reorder targets. Also used as the predicate for filtering rootKeys / getChildren down to the live sets accepted by reorderRoots / reorderChildren.
Implementation
bool isPendingDeletion(TKey key) => _isPendingDeletion(key);