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/common/CachedImage.svelte

@@ -15,7 +15,7 @@
      * Called once the bitmap is decoded, with its intrinsic pixel size. Lets a
      * layout that sizes boxes from artwork (the mosaic) use the shape the image
      * actually has rather than the one its item type suggests.
-     * TRACES: UR-075 | DR-163
+     * TRACES: UR-075 | DR-172
      */
     onNaturalSize?: (width: number, height: number) => void;
   }