feat(archive/imo): formalize IMO 1981 problem Q3 (#4599)
Determine the maximum value of `m ^ 2 + n ^ 2`, where `m` and `n` are integers in
`{1, 2, ..., 1981}` and `(n ^ 2 - m * n - m ^ 2) ^ 2 = 1`.
Co-authored-by: Mario Carneiro <di.gama@gmail.com>
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
