/** Coalesce concurrent work for the same key into one in-flight promise. */
export function sharedInflight<T>(
  inflight: Map<string, Promise<T>>,
  key: string,
  start: () => Promise<T>,
): Promise<T> {
  const existing = inflight.get(key);
  if (existing) return existing;
  const promise = start().finally(() => {
    if (inflight.get(key) === promise) inflight.delete(key);
  });
  inflight.set(key, promise);
  return promise;
}