Given the head of a doubly linked list sorted in non-decreasing order and an integer target, find all pairs of nodes whose values add up to target — each node used in at most one pair — and return the pairs. Because the list is already sorted, this is the linked-list version of the classic "two-sum on a sorted array" two-pointer trick — except walking backward from the tail uses a real .prev pointer instead of an index, which is exactly the kind of operation a doubly linked list makes cheap that a singly linked list can't do at all.