reserve

Earmark up to maximumCost service units against this peer's holdings at leaf leaf, returning a ReservationId to complete against, or null if the peer's available holdings (holdings minus outstanding earmarks) at leaf cannot cover it. The earmark is local state, not replicated (design §4.4): the reserved units simply stay outstanding on the leaf edge until spent, which the accounting already charges. A crashed peer strands its earmarks (design §8.1).