test(03-02): add failing tests for word mode typing engine

- Tests for createWordExerciseState, getCurrentWordIndex
- Backwards compatibility test for createExerciseState mode field
- handleKeyPress test with word-mode state

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/game/typing.test.ts

@@ -2,6 +2,8 @@ import { describe, it, expect } from "vitest";
 import {
 	generateLetters,
 	createExerciseState,
+	createWordExerciseState,
+	getCurrentWordIndex,
 	handleKeyPress,
 	getCurrentLetter,
 	isExerciseComplete,
@@ -172,3 +174,83 @@ describe("isExerciseComplete", () => {
 		expect(isExerciseComplete(state)).toBe(true);
 	});
 });
+
+describe("createExerciseState backwards compatibility", () => {
+	it("sets mode to letters with no words/wordBoundaries", () => {
+		const state = createExerciseState(1);
+		expect(state.mode).toBe("letters");
+		expect(state.words).toBeUndefined();
+		expect(state.wordBoundaries).toBeUndefined();
+	});
+});
+
+describe("createWordExerciseState", () => {
+	it("creates state with letters from words joined", () => {
+		const state = createWordExerciseState(["eis", "die"]);
+		expect(state.letters).toEqual(["e", "i", "s", "d", "i", "e"]);
+	});
+
+	it("stores words and wordBoundaries in state", () => {
+		const state = createWordExerciseState(["eis", "die"]);
+		expect(state.words).toEqual(["eis", "die"]);
+		expect(state.wordBoundaries).toEqual([0, 3]);
+	});
+
+	it("sets mode to words", () => {
+		const state = createWordExerciseState(["eis"]);
+		expect(state.mode).toBe("words");
+	});
+
+	it("sets intervalMs to 0 for word mode", () => {
+		const state = createWordExerciseState(["eis"]);
+		expect(state.intervalMs).toBe(0);
+	});
+
+	it("starts with 0 correct, 0 errors, 0 streak, index 0", () => {
+		const state = createWordExerciseState(["eis"]);
+		expect(state.currentLetterIndex).toBe(0);
+		expect(state.totalCorrect).toBe(0);
+		expect(state.totalErrors).toBe(0);
+		expect(state.correctStreak).toBe(0);
+	});
+});
+
+describe("handleKeyPress with word-mode state", () => {
+	it("works the same way for word-mode state", () => {
+		const state = createWordExerciseState(["eis"]);
+		const result1 = handleKeyPress(state, "e");
+		expect(result1.correct).toBe(true);
+		expect(state.currentLetterIndex).toBe(1);
+
+		const result2 = handleKeyPress(state, "x");
+		expect(result2.correct).toBe(false);
+		expect(state.currentLetterIndex).toBe(1);
+
+		handleKeyPress(state, "i");
+		const result3 = handleKeyPress(state, "s");
+		expect(result3.exerciseComplete).toBe(true);
+	});
+});
+
+describe("getCurrentWordIndex", () => {
+	it("returns which word the letter belongs to", () => {
+		const state = createWordExerciseState(["eis", "die"]);
+		// Index 0 -> word 0 ("eis")
+		state.currentLetterIndex = 0;
+		expect(getCurrentWordIndex(state)).toBe(0);
+		// Index 2 -> word 0 ("eis")
+		state.currentLetterIndex = 2;
+		expect(getCurrentWordIndex(state)).toBe(0);
+		// Index 3 -> word 1 ("die")
+		state.currentLetterIndex = 3;
+		expect(getCurrentWordIndex(state)).toBe(1);
+		// Index 5 -> word 1 ("die")
+		state.currentLetterIndex = 5;
+		expect(getCurrentWordIndex(state)).toBe(1);
+	});
+
+	it("returns 0 for letters-mode state (no boundaries)", () => {
+		const state = createExerciseState(1);
+		expect(getCurrentWordIndex(state)).toBe(0);
+	});
+});