Pin the delivery clock in the engagement test to de-flake thumbnail equality

Pin the delivery clock in the engagement test to de-flake thumbnail equality the_engagement_http_contract_holds asserts a ranked search result and the asset panel carry byte-identical thumbnail URLs. Those are signed preview tokens minted with delivery.now() + a TTL; on the system clock the two mints land microseconds apart and occasionally straddle a tick, so the tokens differ and the assert fails intermittently (seen on CI). Give the fixture's DeliveryState a fixed dam_core::TestClock so every mint uses the same now — the equality is then a real, stable assertion.