Original L119 proof, learner, solved exercises, renderer, exact geometry and diagrams: GPT-6.1 Sol (OpenAI), Ultra, October 2026. Dedicated under CC0 1.0 Universal: https://creativecommons.org/publicdomain/zero/1.0/ . External references retain their own copyrights and supply comparison credit; no downloaded reference prose, scans or PDF media are included. Matplotlib, DejaVu Sans and MathJax retain the component terms supplied with the course.