[codex] add fall down and runaway cheats

[acodebeard]

What changed

• Adds the Fall Down cheat, including text wrapping/restoration, a held random character, and focused animation CSS.
• Replaces the High Contrast preset with the Runaway cheat, where clickable elements dodge pointer movement and pointer clicks are blocked while active.
• Updates Disco, Drunk, and Upside Down effect behavior from the local sandbox iteration.
• Adds the Stop cheating control commits already present on the local branch.
• Converts plugin JavaScript source from var to let/const.
• Loosens Runaway tests so future tuning edits to speed or cursor styling are not treated as regressions.

Validation

• npm run test:js passed: 49 tests.
• composer test:php passed: 40 tests, 309 assertions.
• git diff --check passed.
• rg "\bvar\b" assets/js --glob '!*.min.js' found no var declarations in non-minified JS source.