Repository-backed datasets

StudyDatasets creates short-lived OriginalSampleDataset, RealAnomalyDataset, SyntheticAnomalyDataset and HybridSampleDataset views over repository records. Original views can filter has_anomaly and is_annotated. They do not scan folders or align files by basename. Dataset objects are not persistent state of HybridDataGenerator; callers choose explicitly whether a view should load arrays into RAM.