~x-ai/grok-latest → B (2:3)

Side B changes lasting product behavior by putting each unranked sibling into its own SiblingNavGroup (instead of one combined unranked bucket) and hardens that with a dedicated unit test; Side A is almost pure structural churn—deleting the leftover 1405-line forum.rs monolith and re-homing the identical logic into forum/{mod,feed,views,...}.rs—so it removes module ambiguity but adds no new functionality or bugfix beyond file layout.