Create a routed Tumbler quote

Requires the buyer to join Tumbler first, then creates a durable simulated quote for a routed task. The response also includes match_id and choice_set_id (the same nullable cs_… string): the id of the persisted decision-time choice-set snapshot of the simulated ranked set (rail tumbler, selection layer quote_lock, linked to the returned quote). Choice sets are behavioral observability only — they never gate, rank, price, or settle anything, and capture failures never affect the request.