Download campaign transcripts

Streams one bounded UTF-8 CSV page with one row per campaign attempt. The default and maximum page size is 500. Follow X-Next-Cursor with the same limit until that header is absent; each cursor holds the first request’s upper watermark, so later attempts do not enter the download. Each row carries recipient_id, attempt_no, the accessible call_id, transcript_state, the selected transcript result id and readable Speaker: text lines when ready. A live call with no selected layer is pending; a terminal call with no layer is absent; no accessible call, or an unreadable or unverifiable selected layer, is unavailable. A hidden call has no call id or content. A row-specific object read failure produces unavailable and does not truncate later rows. A broken HTTP transfer is incomplete: discard that page and retry it with the same cursor, or without a cursor for the first page. The read never starts, retries or repairs transcript work and never emits destinations, audio or signed URLs.