ReservationsTrait
constReservationsTrait:Trait<() =>Record<string,ReservedAmounts>>
On an entity, each cost a reservation holds back, by the reservation’s
key: what commitCost spends and refundCost returns of that key
alone. Each resource’s reserved is the total of every key’s hold.
Predicted, beside the resources it holds back, so a correction restores
both together; on anything but a player’s character, room state.