openai/gpt-chat-latest → A (9:1)

Side A replaces the pervasive `CanonicalItemUrl` string wrapper with a structural `ItemId` type, introduces dedicated `item_id.rs`/`item_wire.rs` modules, and propagates the new identity model through reducers, ranking, routing, APIs, HTML, and tests. Side B is a focused correctness change that makes `thread_post_index` consistently 0-based and asserts an invariant with `expect`, but its impact is limited compared with the project-wide architectural improvement in Side A.