feat: chapter select system, multi-chapter support, scene manager refactor, and docs update

public/scenes/demo.json

@@ -5,6 +5,29 @@
     "courage": 0,
     "investigation": 0
   },
+  "chapters": [
+    {
+      "id": "ch1",
+      "label": "第一章：醒来",
+      "startScene": "intro",
+      "thumbnail": "/images/ch1.jpg",
+      "defaultVariables": { "trust": 50, "courage": 0, "investigation": 0 }
+    },
+    {
+      "id": "ch2",
+      "label": "第二章：调查",
+      "startScene": "desk_detail",
+      "thumbnail": "/images/ch2.jpg",
+      "defaultVariables": { "trust": 60, "courage": 10, "investigation": 1 }
+    },
+    {
+      "id": "ch3",
+      "label": "第三章：终局",
+      "startScene": "qte_success",
+      "thumbnail": "/images/ch3.jpg",
+      "defaultVariables": { "trust": 70, "courage": 20, "investigation": 2 }
+    }
+  ],
   "scenes": {
     "intro": {
       "id": "intro",