Function fan_out 

pub(crate) async fn fan_out<I, T, F, Fut>(
    items: Vec<(String, I)>,
    deadline: Duration,
    max_concurrency: usize,
    op: F,
) -> Vec<(String, FanOut<T>)>
where
    I: Send + 'static,
    T: Send + 'static,
    F: Fn(I) -> Fut + Clone + Send + 'static,
    Fut: Future<Output = Result<T>> + Send + 'static,

Run op for each (id, input) under a concurrency cap and a per-item deadline, returning outcomes paired with their id in the input order.

No input id is ever dropped: a task that panics is reported as Failed. This backs SPEC-0014 R33 (per-workspace deadline) and R37 (failures surfaced, never silently removed). It is generic over the op so the timeout/concurrency behavior is unit-tested with injected futures — no real store required.

Input ids must be unique: outcomes are keyed by id, so duplicate ids collide.