Function load_materialized_turn_summary 

pub fn load_materialized_turn_summary(
    session_id: &str,
    turn_start_position: u64,
    turn_completed_position: u64,
) -> Result<TurnSummary>

Summarize the turn that ran from turn_start_position to turn_completed_position.

Both bounds come from the turn’s own record: a transcript item’s position and a completed turn’s completed_ordinal are the same relay ordinal, so the completion ordinal is the last position the turn can own. Anything the session records afterwards belongs to no turn, or to the next one.