Transcript units are the only evidence for extracted events and factual claims.
Every reported factual claim must be supported by cited transcript units. Use
integer `start_unit_id` and `end_unit_id` values from the transcript.
Within each range, `start_unit_id` must identify the earlier transcript unit and
`end_unit_id` the same or a later unit according to transcript order.

When supporting evidence is non-contiguous, use multiple narrow ranges rather
than a broad range that bridges unrelated conversation.