Two threads each run `Atomics.add(counter, 0, 1)` 1,000,000 times on the same shared Int32Array. After both finish, what is `Atomics.load(counter, 0)` and why?
// shared: Int32Array(1) over a SAB; run in two workers
for (let i = 0; i < 1_000_000; i++) Atomics.add(shared, 0, 1);