- generateLetters: random 8-12 letters from level key set
- handleKeyPress: correct/wrong input with adaptive tempo
- Adaptive tempo: 3s start, -200ms after 2x correct, +500ms on error
- Bounds: min 1500ms, max 4000ms
- 18 passing tests covering all behavior
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>