Support traditional data layouts more cleanly

Enable 1D folding in unit-stride via a straightforward mechanism.
First version might require alignment and padding.
Optionally allow unaligned vectors, no padding, etc.

The issue names 1D folding in unit-stride layouts but no files, tests, or entry points. Start by locating the existing folding and unit-stride implementation, then clarify the required alignment and padding behavior. Done should include support for the agreed traditional layout, with the scope of unaligned vectors explicitly decided.

Written by the indexing model from the issue text.