ergodic 0.2.1

Fixed

• WeltLearner fits a slot with no feature columns locally. An empty adjustment set
  leaves nothing for a model to learn, so the nuisance is the training mean (or the
  class frequencies for classification): no upload, no paid fit, no charge to
  max_fits, and predictor_id_ is None. In 0.2.0 it raised, so the "Discover in
  Welt, estimate in ergodic" guide stopped whenever Welt's Arrow found no confounders
  for the treatment, as it does on the guide's own data today. With no adjustment
  variables, DML is then the cross-fitted regression slope of the outcome on the
  treatment, identical to DML with ergodic's ols learner.
• ArcStats treats two NaN gaps (START and END arcs) as equal and hashes them alike,
  so process_map(log) == process_map(log) holds on Python 3.13 and 3.14, where
  dataclass equality compares fields one by one.