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/routes/+page.svelte

@@ -118,7 +118,7 @@
   // The shortcut strip is a mosaic row: one height, each tile as wide as its own
   // artwork. It used to force 16:9 on everything so square music covers lined up
   // with wide backdrops — which lined them up by cropping the covers.
-  // TRACES: UR-075 | DR-163
+  // TRACES: UR-075 | DR-172
   const LIBRARY_STRIP_HEIGHT = 132;
   const libraryTiles = $derived(
     shortcutLibraries.map((lib) => ({ key: lib.id, ratio: assumedLibraryRatio(lib), library: lib }))