Factory for the cross-validated (or in-sample) fitness used by
optimize_panel() by default. Sits alongside the validation,
rotating-validation, and LOCO factories (R/fitness_validation.R,
R/fitness_rotating.R, R/fitness_loco.R) as the fourth fitness strategy:
this one trains and scores on the same (optionally k-folded) data rather
than a held-out partition. Split out of optimize_panel() to keep the main
orchestration function readable.