Asks a store which of its tiers can be reached.
A verify pass has to be able to report a tier it could not read, and the alternative to a contract like this one is a class check on the provider - which the engine deliberately never does, because a provider is chosen by configuration and a branch on its class makes an add-on a special case.
A store with one destination answers with one reachable tier, so a caller holding this needs no branch of its own.
tierStatus() : array<int,string|null>
Why each tier cannot be reached, if it cannot.
Probing is what a status page and a verify pass need, so it must not throw: a tier that cannot even be built is reported as unreachable with the reason rather than taking the caller down.
Tier index keyed to the reason it is unreachable, or NULL when it is reachable.