openai/gpt-chat-latest → B (4:1)

Side B makes a substantial architectural change by replacing query-parameter navigation with canonical `/~/...` browse routes, centralizing URL generation/parsing in `ItemId` (`browse_href`, `from_browse_uri`), updating routing, and adding integration tests for the new behavior. Side A mainly tightens an invariant by replacing a silent fallback with `expect`, aligns documentation/tests around 0-based thread indices, and always renders the post link, which is useful but much narrower in lasting impact.