`revision` is the commit before the index was added, so comparing it to the
repo head always reports stale. The test is whether the case list or any
s*/segmentations/heart.nii.gz blob moved since `revision`.
Add empty-mask index for the heart structure (412 of 616 cases all-zero)
empty_masks.json (schema medotter/empty-masks/v1) lists which s#### cases carry an
all-zero segmentations/heart.nii.gz, measured at revision 997e49fd with
np.count_nonzero(nib.load(f).get_fdata()) == 0 over all 616 heart files.
It lets a consumer decide cohort membership from metadata instead of downloading
616 volumes; EMPTY_MASKS.md documents the fields, the method and when to recompute.
No existing file is touched.
