With a star athlete in their delegation and more than 30 rival nations boycotting the Games, Sri Lanka would have faced its best chance at a medal during the 1976 Olympics. Instead, the country’s delegation unexpectedly withdrew its 13-member team from the Games at the last minute, offering no official reason, and costing marathon runner S.L.B. Rosa — then ranked 11th in the world — what would have been the peak of his career.