docs/DESIGN.md §5d lists one class per line, so retiring one class never conflicts with retiring another #4888

Intent: stop design pulls dying on merge conflicts. Pulls #4644 and #4723 were closed because main moved under them in docs/DESIGN.md: every class in a §5d group sits on one long line, so any two retirements edit the same line.

Done:

• [ ] in docs/DESIGN.md §5d, each group's classes ("style/content.ts, page furniture" first, and every other group with more than one) become a sub-list with one .class code span per line. Each group's lead sentence and every class are kept; nothing else in the file changes
• [ ] registeredClasses() and the §5 named-class reader in scripts/design-lint.ts return exactly the same sets before and after. A test asserts it against a fixed list, or the reader is adjusted in the same pull if it needs to be
• [ ] AGENTS.md (or the §5d lead) says: to retire a class, delete its line

Files: docs/DESIGN.md, scripts/design-lint.ts (only if needed) and one test. Run bun scripts/design-lint.ts and bun test test/design-lint.test.ts.