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/MosaicTile.svelte

@@ -7,7 +7,7 @@
   lines would no longer line up with its neighbours. Keeping everything inside
   the box is what lets `layoutMosaic` own the geometry completely.
 
-  TRACES: UR-075 | DR-163
+  TRACES: UR-075 | DR-172
 -->
 <script lang="ts">
   import type { Snippet } from "svelte";