R(12,12) ≥ 1658: a colouring of K_1657, a Lean proof and computer checks

A red/blue colouring of the complete graph on 1657 vertices with no monochromatic K_12, so R(12,12) ≥ 1658. It consists of the Mathon colouring M(797) and 61 added vertices, each a copy of a certificate vertex or constant on the rest of M(797). Checked by an exhaustive computer search and by a clone-projection argument, and proved in Lean 4 with Mathlib; the compiled proof was also accepted by leanchecker and by the independent kernel nanoda. The work was done with Claude (Claude Opus 5.5, Anthropic), starting from exploratory work by GPT-6 Astra (OpenAI); see the README.