Confirmed cross-venue arbitrage opportunities (v2)

Confirmed, engine-fired Opportunities with one exact selected execution leg per outcome. Money fields name USD/contract units, include venue fees under the disclosed conservative_per_contract model, and distinguish payout edge from modeled ROI. Every row is rechecked against the current matched market before serving; stale or malformed rows fail closed. This is not a Market object or /v2/markets filter.