After a deploy a browser could keep the old entry document, which named a
bundle that no longer exists, and the missing script was answered with the
entry document, so the page was blank with only a MIME-type error to say why.
The hashed bundle is immutable and a missing file is a 404; the entry document,
the worker, the manifest, the icons and the theme's colors.css are revalidated.
The pages carry a content security policy. The service worker fetches its shell
past the HTTP cache, logs a failed install, refreshes the cached entry document
on each navigation and holds cache writes open with waitUntil. The proxy test
pins the cache rules, the 404s and the policy; docs/swarm/ui.md says what the
browser keeps.
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