catalog.md said content_hash is computed only for import duplicate
detection and reconnection, and left "the same image catalogued twice"
as unspecified. FR-CAT-11a now handles the within-root case: it proves a
group by content_hash where every copy has one, otherwise by first and
last megabyte digests kept in dedup_probes, a table created on first use
rather than by migration (core/dr-catalog/src/duplicates.rs:296). The
cross-root case stays open, and the bullet now says which half is done.
dtourolle
9e099a07ab