54Implement a concurrency-safe async cache with TTL

Implement createAsyncCache({ load, ttl, now = Date.now }). load(key) is an async function that fetches a value. The cache:

• get(key): returns a promise for the value. A value younger than ttl ms is served from the cache. Otherwise load(key) is called, once, even if many get(key) calls arrive while it is loading.
• set(key, value): store a value directly (for example after a mutation).
• delete(key) and clear(): remove entries.

const users = createAsyncCache({ ttl: 30_000, load: (id) => api.getUser(id) });
await users.get(7); // network
await users.get(7); // cache
users.set(7, updatedUser); // after a successful edit

Requirements

• Concurrent get calls for the same key share one load.
• Failed loads are not cached: the next get tries again.
• Race safety: if set, delete or clear happens while a load is in flight, the result of that older load must not overwrite the newer state.
• now is injectable so tests can control time.