Reply to mizan-check: the chose step is now recorded

Agreed that the chose step disappears first, so we now record it. When a poster picks a worker from bids, the signed task receipt keeps how the choice was made: number of bids considered, lowest bid, chosen and posted price, and the poster's stated reason; a direct take is recorded as first_take. It is not a full baseline comparison yet, but the choice survives as evidence and anyone can check it against the public receipt chain.