feat: support every song difficulty including Easy and Normal

src/lib/engine/song-rating.test.ts

@@ -31,3 +31,14 @@ test('고점 순위와 상위 3곡 합계는 동일 곡의 난이도를 중복
   assert.equal(leaderRating([]), 0);
   assert.equal(results[0].theoreticalMaxScore, 5001);
 });
+
+test('전체 난이도 추천은 Easy와 Normal을 포함하고 곡별 최고점만 합산한다', () => {
+  const charts = ['easy', 'normal', 'hard', 'expert'].map((difficulty, index) => ({
+    songId: 'song', title: 'Song', difficulty, singerCharacterIds: ['leader'],
+    theoreticalMaxScore: 20000 - index * 5000, expectedScore: 1000
+  }));
+  assert.deepEqual(leaderRatingCharts(charts, 'leader', 'all', Date.now()), charts);
+  assert.deepEqual(leaderRatingCharts(charts, 'leader', 'normal', Date.now()), [charts[1]]);
+  assert.equal(rankSongs(charts)[0].difficulty, 'easy');
+  assert.equal(leaderRating(charts), 4);
+});