~x-ai/grok-latest → B (3:1)

B makes a lasting design change: recent votes become append-only Vec/List with query-time capping, drops write-path trim and BTreeSet bookkeeping, bumps the projection schema, and adds a regression test. A mostly adds a temporary plan.md plus a thin RouteContext newtype over ThreadNav with no call-site migration, so it is scaffolding rather than shipped substance.