Function load_materialized_transcript_tail 

pub fn load_materialized_transcript_tail(
    session_id: &str,
    limit: usize,
) -> Result<Vec<Arc<TranscriptItem>>>

Read the newest limit transcript items for a session, oldest first.

A conversation view seeds itself from the tail and discards everything before it — ChatState::from_materialized_tail keeps TAIL_SEED_ITEMS and drops the rest — so reading the whole transcript to show the end of it is work proportional to history for a result that never was. On a real session that meant reading 28,066 rows to render 256.

The materialized_transcript_position index covers the ordering, so this costs the rows it returns rather than the rows that exist.