Add three playable puzzle variants in an isolated lab

labs/variants/index.html

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+<!doctype html>
+<html lang="zh-CN">
+<head><meta charset="utf-8"><meta name="viewport" content="width=device-width,initial-scale=1"><title>玩法变种 · HEX TRAILS Lab</title><link rel="stylesheet" href="./style.css"></head>
+<body><main>
+<header><a href="../../">← 返回主线</a><span>HEX TRAILS / 独立实验</span></header>
+<h1>同一条轨道，三种新玩法</h1><p class="intro">沿着弯轨出发，转动底盘寻找新的可能。</p>
+<nav class="modes" aria-label="玩法"><button data-mode="color">01 分色归站</button><button data-mode="switch">02 机关换轨</button><button data-mode="relay">03 接力停靠</button></nav>
+<section class="brief"><h2 id="title"></h2><p id="rule"></p></section>
+<div class="level-row"><label>关卡 <select id="level"></select></label><span id="count"></span></div>
+<div class="board-wrap"><canvas id="board" aria-label="六角轨道棋盘，点击箭头出发"></canvas></div>
+<p id="message" role="status" aria-live="polite"></p>
+<div class="controls"><button id="left">↶ 左转</button><button id="undo">撤销</button><button id="hint">提示</button><button id="reset">重来</button><button id="right">右转 ↷</button></div>
+<button id="next" hidden>下一关 →</button>
+<details><summary>键盘操作 / 棋子列表</summary><div id="pieces"></div></details>
+<footer>旋转时棋子位置与箭头不动。交叉不换轨，试路免费。<br>实验共 15 关 · 不写入主线进度</footer>
+</main><script type="module" src="./app.js"></script></body></html>