Cover the complete provided transcript from its first source unit to its last
source unit. Return scenes in source-unit order with no gaps or overlaps. Use
only positive integer source-unit IDs from the transcript, and give every scene
one inclusive `start_unit_id` and one inclusive `end_unit_id`.

Return exactly one JSON object and no explanatory text. The object must contain
only a non-empty `scenes` array. Each scene object must contain only
`start_unit_id` and `end_unit_id`.