Open kehto's verified-artifact cache over Cache Storage when the page has
it, and nothing otherwise. Cached bytes are re-hashed on every read by
resolveNapplet, so the cache accelerates and never authenticates.
Returns Promise<NappletArtifactCache|undefined>
The cache, or undefined where Cache Storage is unavailable.
Open kehto's verified-artifact cache over Cache Storage when the page has it, and nothing otherwise. Cached bytes are re-hashed on every read by
resolveNapplet, so the cache accelerates and never authenticates.