fix: return None for corrupt images, add collate_skip_none, remove RandomGrayscale/RandomAffine (audit #10)

fix: lower image size filter from 64px to 16px minimum

fix: use as_posix() for manifest paths to avoid Windows backslash bug

feat: complete — trained EfficientNet detector, centroids, integrated into ensemble

feat: add EfficientNet training script — reads manifest.csv, trains binary classifier