First Lesson at a Level
`levels` is a list of lesson difficulty levels, sorted in ascending order (the same level may repeat). Given a `target` level, return the index of its FIRST occurrence using binary search, or -1 if no lesson has that level.
`levels` is a list of lesson difficulty levels, sorted in ascending order (the same level may repeat). Given a `target` level, return the index of its FIRST occurrence using binary search, or -1 if no lesson has that level.
