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/library/+page.svelte

@@ -45,7 +45,7 @@
   // The overview is a mosaic: rows of one height, tiles of their own widths, so
   // a square music cover sits beside a wide backdrop without either being
   // cropped to the other's shape. Each category also gets a favourites tile of
-  // its own, beside the library it belongs to. TRACES: UR-075 | DR-163, DR-164
+  // its own, beside the library it belongs to. TRACES: UR-075 | DR-172, DR-173
   const mosaicEntries = $derived(buildLibraryMosaic(visibleLibraries));
 
   // Track if we've done an initial load and previous server state
@@ -255,7 +255,7 @@
              category's own favourites sits beside its library — a labelled tile
              at the same weight as a library is the difference between a feature
              people find and one they don't. ux-flows §5C.2.
-             TRACES: UR-067, UR-075 | DR-117, DR-163 -->
+             TRACES: UR-067, UR-075 | DR-117, DR-172 -->
         <MosaicGrid items={mosaicEntries} gap={8}>
           {#snippet tile(entry)}
             {#if entry.kind === "favorites"}