fix: clippy lint and multi-spacer distribution

- transcript.rs: use sort_by_key with Reverse instead of sort_by
  (clippy::unnecessary_sort_by)
- flex.rs: distribute extra space proportionally among multiple spacers
  instead of picking a single winner
- .gitignore: add .DS_Store entries

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,5 @@
 # bv (beads viewer) local config and caches
 .bv/
 target/
+.DS_Store
+src/.DS_Store