Bundles the fitted FeatureTransform, a calibrated classifier, and a ModelCard
into one object that saves/loads as a single file and predicts identically on
raw rows after a reload. Purely additive over 2.0.0 -> MINOR bump.

- artifact.py: TrainedModel, ModelCard, train_artifact (isotonic calibration
  via CalibratedClassifierCV over a FrozenEstimator)
- joblib added as a dependency
- README: artifact usage + version history (2.1.0 is the last package version)