Get one namespace, with its size

One namespace and the number of memories and chunks it holds — the count a destructive confirmation has to show. Same native op as the list route, so it costs one billed unit and no egress.

exists: false with zero counts is the answer for a namespace that holds nothing. That is not a 404: a namespace that is already gone is a question with a true answer.

Subject to the credential's scoped_namespaces — otherwise a scoped credential could learn the SIZE of namespaces it cannot read, one name at a time.

Requires the memories:read scope. Accepts all three lanes: a session JWT (bounded by the caller's role on this workspace) or an il_* machine credential whose own scope set, workspace set and validity window cover it — both ceilings apply and neither can raise the other, so a credential carrying memories:read is still refused where the role does not permit it.

A credential with no scope record resolves to LEGACY_DEFAULT, which INCLUDES memories:read — every key minted before scoping keeps working here.