~x-ai/grok-latest → B (3:1)

B fixes real indexing inconsistency by switching rank-history thread_post_index from 1-based-with-silent-0-default to shared 0-based paths (/t/tag/N), replacing unwrap_or(0) with invariant enforcement and updating API/UI/tests/docs accordingly. A only deletes a dead zero-ratio guard already enforced upstream and retunes one unit test, which is sound cleanup but lower lasting impact.