httpx.AsyncClient.get() does not accept proxy= per-request.
Shared client caused every check_one call to raise TypeError,
silently caught as False, so 0/N proxies ever passed.
Reverted to per-proxy AsyncClient(proxy=url) in _check_one.
Semaphore concurrency + wall-clock cap preserved.
Also re-adds _check_one helper that was accidentally removed.
Made-with: Cursor