feat: audio system, demo scene updates, docs, and engine improvements

public/scenes/demo.json

@@ -10,6 +10,10 @@
       "id": "intro",
       "videoUrl": "/videos/intro.mp4",
       "subtitleUrl": "/subtitles/intro.vtt",
+      "bgmUrl": "/audio/calm_bgm.mp3",
+      "bgmVolume": 0.6,
+      "bgmCrossFade": 1.5,
+      "videoMuted": true,
       "choices": [
         {
           "text": "走向左边那扇发光的门",
@@ -126,6 +130,10 @@
     "right_door": {
       "id": "right_door",
       "videoUrl": "/videos/right_door.mp4",
+      "bgmUrl": "/audio/tense_bgm.mp3",
+      "bgmVolume": 0.7,
+      "bgmCrossFade": 2.0,
+      "videoMuted": true,
       "qte": {
         "triggerTime": 1.0,
         "prompt": "躲避飞来的石块！",
@@ -185,6 +193,9 @@
       "id": "stay",
       "videoUrl": "/videos/stay_loop.mp4",
       "subtitleUrl": "/subtitles/stay.vtt",
+      "bgmUrl": "/audio/calm_bgm.mp3",
+      "bgmVolume": 0.6,
+      "videoMuted": true,
       "loopStart": 3.0,
       "loopEnd": 6.0,
       "choices": [