~x-ai/grok-latest → A (3:1)

A finishes a real maintainability win: it deletes large duplicated copies of HTTP/OAuth/assert/build helpers from auth.bb/grants.bb/integration.bb and centralizes them in test.common and test.oauth (including multi-user mock Google, complete-registration!, slug-server-env, cargo build helper). B mainly adds a forward-looking plan.md plus a thin RouteContext wrapper around ThreadNav with no call-site migration of ItemId yet, so most lasting substance is still unfinished scaffolding.