User transformer should allow mapping by code as well as name

Code is consistently more concise, stable, less prone to containing problematic values, and more likely to line up with source data. Although we've always mapped to name, code is the more useful value

Desirable to allow mapping to folio_code as well as folio_name.

Start by locating the user transformer and the existing mapping for folio_name. Trace how transformer mappings are configured and validated, then inspect any relevant tests. Done means the same mapping flow can use folio_code as an alternative to folio_name, with existing name-based behavior preserved.

Written by the indexing model from the issue text.