Ruzsa’s genus-one problem

Found by a randomized valid-set walk starting from witness #122 at modulus 1009. Residue 498 was the least-conflicting direct 20th element (three ordered nontrivial solutions), and the walk repaired those conflicts by a small sequence of swaps to reach this set. Direct enumeration gives exactly 20 solutions to a+3b=2c+2d, all diagonal. A further 5,000,000-step walk did not find a 21st element.