~anthropic/claude-sonnet-latest → A (8:2)

Side A is a substantial, coherent refactor introducing a structural ItemId enum to replace the stringy CanonicalItemUrl newtype, touching reducer, ranking, RPC, and HTML layers consistently with new modules (item_id.rs, item_wire.rs) and updated tests, representing real architectural progress. Side B is a small, useful bugfix (off-by-one/expect vs unwrap_or for thread_post_index) but is narrow in scope and lower lasting impact compared to A's broader identity-model overhaul.