fix: allocate upper list slots on a load only for lists the dump fills

[doubleinfinity]

A load gives a wide list to an upper list at or below the level of its node only if the dump fills that list. A wide list has a descriptor and m + 1 target slots. An empty list of this type is one empty word, as an empty list above the level of its node is. Thus a level in the layer table uses one word for each layer and no slots.

The in-degree pass of the load promotes an empty word to a wide list when an entry at that layer refers to its node. The inbound counter of a list is on its wide descriptor.

Insertion opens a wide list for each layer of a new node up to its level. Insertion promotes an empty word when it first counts into the word. The reverse update puts an entry into a list only after this count. Thus an empty word at or below the level of its node never holds an entry.

The storage that a load makes follows the node count, m and the entries in the dump. A dump that keeps no list empty at or below a level loads to the same graph as before. Search and insertion on a loaded graph give the same results as before.