Function step 

pub fn step(
    state: HostRolloutState,
    event: Event,
    now: DateTime<Utc>,
    policy: &RolloutPolicy,
) -> Result<(HostRolloutState, Vec<Effect>), TransitionError>

Pure reducer. Same signature on agent and CP-mirror sides.

now is a parameter so tests can advance time deterministically and the reducer cannot drift from any caller’s notion of “current time” by hitting a global clock.

policy is borrowed from the signed manifest the caller already verified (via [nixfleet_reconciler::verify_rollout_manifest]). The reducer cannot fetch it from a different source.