~x-ai/grok-latest → B (1:3)

B fixes a real reducer correctness bug by moving the zero-ratio early-return before ensure_item and voted_pairs.insert, so invalid votes no longer create ghost items or mark pairs voted, and updates the test to lock that in. A mainly deletes a leftover monolithic forum.rs and finishes relocating the same logic into forum/*—useful module hygiene, but mostly structural churn without new behavior.