openai/gpt-chat-latest → B (4:1)

Side B makes concrete behavioral changes: it enforces the invariant that a rank-history post must exist in `ingests_by_scope_thread` with `expect(...)` instead of silently falling back to `0`, aligns `thread_post_index` with the actual 0-based `/t/tag/N` routes, updates rendering to always emit the link, and adds integration tests documenting the behavior. Side A mostly introduces a planning document plus a `RouteContext` wrapper around `ThreadNav` and re-exports it, which is useful scaffolding but does not substantially change project behavior yet.