A 3-year-old boy from rural Maharashtra presents with a 3-day history of high fever (39.5°C), cough, coryza, and conjunctivitis. On examination, small white spots with red halos are noted on the buccal mucosa opposite the molars. The child has not received any vaccination. Two days later, a maculopapular rash appears on the face and hairline, spreading downward to the trunk and limbs. What is the most likely diagnosis and which period of the disease does the appearance of Koplik spots represent?