A 5-month-old boy has a tracheostomy due to airway obstruction from choanal stenosis noted at birth. He has also been diagnosed with bicoronal craniosynostosis, and his head shape is brachycephalic, with the anterior fontanelle widely open on palpation. His eyes are widely set and proptotic. He has syndactyly of the second, third, and fourth digits of both hands. Which of the following is the most likely diagnosis?