prove expected bound

Maybe we can "prove" this property, because we have that supply shares are less than total supply shares (safe because in morpho-blue we proved that total supply shares is the sum of all supply shares), and we have that total supply assets is less than 2^128 _Originally posted by @QGarchery in https://github.com/morpho-org/vault-v2/pull/811#discussion_r2544800816_