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.

src/lib/components/library/mosaic.ts

@@ -9,7 +9,7 @@
 //
 // Presentation only — nothing here knows what a library or a media item is.
 //
-// TRACES: UR-075 | DR-163 | UT-158, UT-159, UT-160
+// TRACES: UR-075 | DR-172 | UT-158, UT-159, UT-160
 
 /** A tile to place: an opaque key and the aspect ratio (width / height) to honour. */
 export interface MosaicInput {