‘but there is no legal forum for it.’
IF Cruz wins the primary, you’re in for an eye opener. And it will not come from Trump.
There is no legal forum for CRUZ to bring/begin the legal case. His legal response would be to someone seeking to get him declared ineligible.
There is no forum for him to begin an action to have himself declared eligible.
He is assumed eligible until someone brings an action to have him declared ineligible, and at that time Cruz would fight it.