A stored secret reads back as "****", and a registry write that sent it
back stored "****" as the secret: a webhook's password or signing key,
a store's or directory's password, a DNS provider's key, an OAuth
client's secret. Written straight at a secret it now keeps the stored
one; anywhere else, inside a whole object, a create or a variant switch,
the write is refused with invalidProperties, since there's no stored
secret it could stand for. Accounts keep their own credential checks,
which already did this.