Maybe rework header logic to contain metadata in `size_type`

Then it would be possible to store any number of links of any type.
And the signature `count -> T` would change to `count -> size_type`

Start by reading the header logic and the definitions or call sites for size_type, link storage, and the count -> T signature. Determine how the metadata redesign would support any number of links of any type, and consider the work complete when the proposed representation and signature changes are consistently defined.

Written by the indexing model from the issue text.