# Kaprekar pipeline → 6174 guaranteed ≤7 iterations
def kaprekar_converge(n: int) -> Tuple[int, int]:
    for i in range(7):
        digits = sorted(str(n).zfill(4))
        n = int(''.join(reversed(digits))) - int(''.join(digits))
        if n == 6174: return 6174, i+1
    return n, 7