~x-ai/grok-latest → B (2:5)

B fixes real correctness and permission bugs by anchoring catch-up to durable ingest index (not timestamps), so concurrent/same-ms and clock-rollback posts are not missed, and private-room visibility/revocation cannot leak or skip content; it also adds room on FeedPost and solid multi-user integration tests. A is a solid ranking heuristic refinement (attach-isolate before isolate-isolate, then zip adjacent ranks) with unit tests, but it optimizes pair suggestion quality rather than closing feed reliability/security gaps.