Add or update a member

Upserts the principal's grant. Only an Owner may grant the Owner role — a rule on the request BODY rather than a scope, so no members:write credential can grant Owner unless its holder's role is Owner.

Requires the members:write 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 members:write is still refused where the role does not permit it.

A credential with no scope record resolves to LEGACY_DEFAULT, which does NOT include members:write — mint a key with it explicitly (POST /v2/workspaces/{id}/keys).