feat(03-02): implement word mode for typing engine

- 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>

src/game/typing.ts

@@ -40,9 +40,47 @@ export function createExerciseState(level: number): TypingExerciseState {
 		correctStreak: 0,
 		totalCorrect: 0,
 		totalErrors: 0,
+		mode: "letters",
+		words: undefined,
+		wordBoundaries: undefined,
 	};
 }
 
+export function createWordExerciseState(words: string[]): TypingExerciseState {
+	const letters: string[] = [];
+	const wordBoundaries: number[] = [];
+	for (const word of words) {
+		wordBoundaries.push(letters.length);
+		for (const ch of word) {
+			letters.push(ch);
+		}
+	}
+	return {
+		currentLetterIndex: 0,
+		letters,
+		intervalMs: 0,
+		correctStreak: 0,
+		totalCorrect: 0,
+		totalErrors: 0,
+		mode: "words",
+		words,
+		wordBoundaries,
+	};
+}
+
+export function getCurrentWordIndex(state: TypingExerciseState): number {
+	if (!state.wordBoundaries || state.wordBoundaries.length === 0) return 0;
+	let wordIndex = 0;
+	for (let i = 0; i < state.wordBoundaries.length; i++) {
+		if (state.wordBoundaries[i]! <= state.currentLetterIndex) {
+			wordIndex = i;
+		} else {
+			break;
+		}
+	}
+	return wordIndex;
+}
+
 export function handleKeyPress(
 	state: TypingExerciseState,
 	pressedKey: string,

src/types.ts

@@ -74,6 +74,9 @@ export interface TypingExerciseState {
 	correctStreak: number;
 	totalCorrect: number;
 	totalErrors: number;
+	mode: "letters" | "words";
+	words?: string[];
+	wordBoundaries?: number[];
 }
 
 // Screen names