feat: Stress it beat for Modules 6-10, Module 11 (Break your own engine), level solver tool

- lessons 8 and 9 updated for the once-per-move rule, per-object negated
  conversion and the new ordering tests
- tools/solve-spellgrid.mjs: breadth-first search over a SpellGrid level
  with the TypeScript reference; --never-rule and --naive
- README, harness.md and the mentor skill know the new beat and module

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

package.json

@@ -11,6 +11,7 @@
     "check": "node tools/sync-check.mjs",
     "test:site": "node --test site/scripts/*.test.mjs",
     "check:links": "node tools/check-links.mjs",
-    "play:spellgrid": "node tools/play-spellgrid.mjs"
+    "play:spellgrid": "node tools/play-spellgrid.mjs",
+    "solve:spellgrid": "node tools/solve-spellgrid.mjs"
   }
 }