- Add createWordExerciseState() that splits words into letters with boundaries - Add getCurrentWordIndex() helper for UI word tracking - Extend TypingExerciseState with mode, words, wordBoundaries fields - Backwards compatible: createExerciseState sets mode='letters' - All 85 tests pass Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>