Describe consolidating duplicate originals in the manual and the register

FR-CAT-11a records what #67 built: groups of the same file listed on
demand, proved the same before anything moves, merged onto one survivor
and the rest trashed a group at a time. The manual's new section under
The library says how to reach the page, how the copy that stays is
chosen, what the check reads and what merges.

docs/dev/requirements.md

@@ -179,6 +179,17 @@ card into the library. Both are needed.
 original filename) and by content hash, offering skip or import-as-new. Camera filenames wrap at
 `IMG_9999`, so filename alone is insufficient. Existing catalog duplicates are detectable on demand.
 
+**FR-CAT-11a — Consolidating catalog duplicates.** Files already in the library more than once
+(same root, camera, capture time and size) shall be listed on demand as groups, from the library
+and from Settings. Before anything moves, each group shall be proved the same file by a stored full
+digest or by a digest of the first and last megabyte of every copy, and its copies' develop edits
+compared; a group that differs in either is left out and the review says why. One copy survives —
+not under a backup-looking folder, then camera-named, then oldest, overridable per group — and the
+others' collections, keywords, highest rating, agreed flag and label, and faces are merged onto it,
+with disagreements reported rather than decided silently. Each group is merged and its other copies
+moved to the trash (FR-CAT-15) in one transaction: wholly consolidated or untouched. Nothing is
+deleted.
+
 **FR-CAT-12 — Versions (virtual copies).** An image may carry multiple named `Version`s, each with
 an independent edit graph, without duplicating source data. Versions are creatable, nameable,
 deletable, and independently exportable; one is the default.