sitemap.xml and llms.txt are built at most once a minute, never once per stranger's request #3142

From #2920 (strangers cannot exhaust the forge). GET /sitemap.xml runs a query of up to 5000 rows on every request (sitemapPaths in src/web/standards.ts), and /llms.txt lists repositories per request too. Both send cache-control: public, max-age=3600, but that helps only when an edge cache sits in front, and a stranger can request either one as fast as they like.

Done looks like:

• [ ] Each of the two bodies is built at most once a minute per forge, in process. Use the same WeakMap<App, Fresh<…>> pattern auditCache uses in src/web/pages/og.ts. Concurrent requests share one build.
• [ ] A new public repository appears within a minute, and a repository made private is gone within a minute. Never list a private repository, even briefly from the cache. Making a repository private clears the cache.
• [ ] Tests: 50 requests in a row build the body once (count the builds through a seam, not with a wall clock). A repository turned private drops out of the next response.

Files: src/web/standards.ts, a test beside the existing standards test.

Parked by the crew: the check failed ("a suspended account's repositories leave every list > suspended: a stranger and a reader find them nowhere") and the decider saw no point in another try (pull #3146 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.