A 16-way trie over the address, used when the address space is too large to give every address a slot number in a flat array. It is grown in place and never copied: a slot is filled once and never repointed, which is what removes the per-write path copy the `Map` paid. A `Leaf` may sit above its natural depth - that path compression is the whole reason this is a union rather than a fixed-depth array of arrays, since depth follows from the address width and would otherwise be known - so the leaf carries its own address and it is checked on arrival.