fix(hub): serve the SPA under a fingerprint of what it is

`Cache-Control: no-cache` requires a browser to revalidate, but it only binds
one that asks. A browser that cached app.js before that header existed applies
heuristic freshness instead — a fraction of the file's age, which for a file
dated weeks ago is days — and never asks. It then runs an old player against a
new node.

That cost most of a session. A phone kept a player without the read-ahead bound
and filled the browser's buffer ceiling at 106 MB, the exact symptom the bound
had been written to remove, for an hour after the bounded player went live. A
fix that is written, tested, deployed and served, and still not what runs, is
indistinguishable from a fix that does not work.

The whole module graph now lives under `/a/<content-hash>/`. A path prefix
rather than a query string, because relative imports inherit it: `app.js`
reaching for `./i18n.js` gets the build it was written against, and never a
mixture of two — which does not render a stale page, it fails to link. The URL
changes with the content, so those may be cached hard.

`sw.js` stays at the root. Its scope is its own path, and under the prefix it
would no longer control the pages whose downloads it exists to intercept.

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