The co-op fits a tree, a margin classifier, and a small net on the same training slice, then scores them on a validation slice against the agreed metrics and keeps the best pair of algorithm plus training data. Which workflow step is that comparison?

Fitting a tree, a margin classifier, and a small net on the same training slice, then scoring them on a validation slice and keeping the best pair, is evaluate-the-model. That is metric scoring on the validation set. It is not the later independent test and not a kiosk re-pack.