Re-verify Teikari article quotes and GDC speaker, soften unsourced
claims (lessons 01, 06, 07, 10), add course-simplification bullets to
lesson 07's differs box, qualify OPEN obstruction wording in research,
and add tools/check-links.mjs (report-only) with npm run check:links.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>