docs: say what the one-liner trusts, and how to check it yourself A page that answers the question honestly rather than asking for trust: what the installer trusts, what each verification tier proves, what "unavailable" means as against "failed", and the two-origin check (script from GitHub, checksum from our cluster) that is worth more than a checksum published beside its own file. It also says what is NOT covered: the host's own dependency chain, a compromise of both origins at once, and that the binaries are not yet byte-reproducible.