ENHANCEMENT: Crossover design studies supported by chevron

How will crossover studies be able to use `chevron`? The treatment groups in these studies are controlled by a meta dataset `ATX` rather than `ADSL`.

Start by tracing how chevron currently obtains treatment groups from the ADSL dataset, then compare that flow with the crossover-study requirement for the ATX meta dataset. Define the expected ATX inputs and treatment-group behavior before identifying the affected implementation and tests; the issue does not name specific files or tests.

Written by the indexing model from the issue text.