feat: 调整喜鹊位置并更新 PDF 导出

- 删除左上角喜鹊，左下喜鹊右移 3 格，右上喜鹊左移 1 格
- PDF 默认输出改为封面demo.pdf
- 重新生成 PDF 样例

assets/js/scene.js

@@ -39,11 +39,10 @@
 
   /* 飞鸟：数量减少，间距不等，仅少量成对；上下拉开分布 */
   const birds = [
-    { x: 0, y: 2, kind: 'magpie' },
-    { x: 26, y: 2, kind: 'magpie' },
+    { x: 25, y: 2, kind: 'magpie' },
     { x: 1, y: 4, kind: 'sparrow' },
     { x: 3, y: 8, kind: 'crow' },
-    { x: 6, y: 13, kind: 'magpie' },
+    { x: 9, y: 13, kind: 'magpie' },
     { x: 9, y: 15, kind: 'sparrow' },
     { x: 26, y: 5, kind: 'crow' },
     { x: 17, y: 9, kind: 'sparrow' },