Opaque, underdocumented Google-operated cache (served under web-pkg-c.com) that stores Signed Exchange packages fetched from origins. Prefetches them to users from top ~3–5 search results. Storage is not unlimited, so old and sometimes even fresh entries get evicted — producing CORS-looking errors in DevTools that actually mask sub-resource cache misses. Took Pokrywka weeks of black-box testing because its behavior is underdocumented and Cloudflare serves a different response to Googlebot than to humans.