Function step 

pub fn step(
    record: RolloutRecord,
    event: RolloutEvent,
    now: DateTime<Utc>,
) -> Result<(RolloutRecord, Vec<RolloutEffect>), RolloutTransitionError>

Pure rollout-level reducer. CP-side only (rollout events are synthesized from inbound agent events; agents never emit RolloutEvents).

now is a parameter (no chrono::Utc::now() inside the crate); the applier supplies it from its own ClockHandle so test fixtures with FakeClock stay deterministic.