The solver's position key reads stackAt (each tile oldest first). New tool tools/verify-levels.mjs checks course/10-spellgrid/levels/proofs.json: each level's shortest solution, and that it exhausts without its idea (neverRule) or on the naive engine. Levels 4-10, one course idea each, with test_10_level_N_solution in both tracks, scenarios and Lesson 10's list; l1 no longer matches l10, and proofs.json is not listed as a level. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>