A 3-year-old boy is brought to the pediatric ophthalmology clinic by his mother with a complaint of inward deviation of the right eye, which she first noticed at age 2 years. On examination, the child has a visual acuity of 6/6 in both eyes. The right eye is deviated medially at rest. Cover test shows that when the right eye is covered, the left eye makes an outward movement to fixate, and when the left eye is covered, the right eye moves outward to fixate. The deviation measures 35 prism diopters on the cover test. Extraocular motility is full in all gazes. The child has no history of trauma, neurological symptoms, or systemic illness. What is the most likely diagnosis?