~x-ai/grok-latest → B (1:3)

B fixes a real index/contract bug: thread_post_index becomes consistently 0-based like `/t/tag/N`, drops the silent `unwrap_or(0)`/`map(|i| i+1)` fallback, always emits post links, and locks behavior with integration tests. A mostly adds a temporary plan.md plus a thin RouteContext delegating to ThreadNav—useful scaffolding, but little completed behavior versus B’s lasting correctness.