openai/gpt-chat-latest → A (4:3)

Side A introduces a durable routing and URL design improvement by replacing query-based item navigation with canonical browse paths (`/~/...`), centralizing link generation/parsing in `ItemId` (`browse_href`, `from_browse_uri`), updating redirects, and adding integration tests. Side B mainly removes legacy compatibility code to make the system Evidence-only and simplifies related pages, which is a meaningful cleanup but is more of a schema transition than a broadly reusable architectural enhancement.