Merge branch 'master' into worktree-mosaic-library

Renumbers the mosaic's requirement IDs out of the way of the download work
that landed on master in parallel: it had already claimed DR-163/DR-164 and
UT-162, so the mosaic layout is now DR-172, the library favourites scope
DR-173, and its composition test UT-167.

Note for the download branch: its UT-162..UT-165 rows trace to DR-163..DR-166,
none of which are defined in requirements.md — that branch defined DR-167..171
instead. Those references are orphaned and want a look; nothing here touches
them.

docs/specs/library-mosaic.md

@@ -1,7 +1,7 @@
 # Spec: Library mosaic (library overview + home shortcuts)
 
 **Status:** Implemented
-**Requirements:** UR-075 → DR-163, DR-164 (with UR-067 → DR-117 extended)
+**Requirements:** UR-075 → DR-172, DR-173 (with UR-067 → DR-117 extended)
 **UX spec:** [ux-flows.md](../ux-flows.md) §5C.2 (Favourites)
 
 ## Summary