Rename PredictionMetadata (prediction_metadata.py) to MongoActivities (mongo.py) and move update_simple_metrics_adaptive_state there from ModelMetrics, so both Mongo-backed activities share one class and repository (PR #59 review, agreed with the class author). write_prediction_metadata is unchanged; the pure hysteresis helpers stay in model_metrics.py. References in activities.py, docstrings and tests updated.
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