feat: 新增 page3 演职员表并拆分多页资源

- 新增 page3.html：无「天」背景的演职员表与火山场景
- 重构为页面专属 CSS/JS 与共用的 text-scene 渲染引擎
- PDF 导出支持 --page 参数

page1.html

@@ -0,0 +1,18 @@
+<!DOCTYPE html>
+<html lang="zh-CN">
+<head>
+<meta charset="UTF-8">
+<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">
+<meta name="color-scheme" content="dark">
+<title>封面</title>
+<link rel="stylesheet" href="assets/css/common.css">
+<link rel="stylesheet" href="assets/css/page1.css">
+<script src="assets/js/text-scene.js" defer></script>
+<script src="assets/js/page1.js" defer></script>
+</head>
+<body>
+
+  <div class="scene"></div>
+
+</body>
+</html>