live: true no longer waits, and every collector serves it

Asking for a live read used to hold your call open while a dedicated collector long-polled for it - and held two collectors long-polling around the clock for the sixteen such calls a week. Now live: true means: answer from a read no older than the API's own cache if one is in hand; otherwise queue one priority fetch for the next collector that checks in and answer at once from the record, with live_status saying it is pending and when to call again. A subject with nothing recorded answers live_pending. Collectors check in instead of polling, the door tells each one when to come back, and any collector in the fleet picks up a live fetch first. Receipts now say what a fetch was worth - new facts, ingest time, bytes read - and a collector earns a point only for a fetch that returned data.