test: 零依赖对拍测试台（可在不改 dist/ 的前提下复现 WE 行为）

tools/serve.mjs    本地静态服务 + 测试驱动注入（模拟 WE 下发用户属性、冻结动画相位、藏 canvas 取参考帧）
tools/cdp.mjs      最小 CDP 客户端（真实时间等待；--screenshot 的虚拟时间会在骨骼加载完成前就耗尽）
tools/shot.mjs     截图/取状态，就绪判据可传入
tools/diff.mjs     ffmpeg 差分：PSNR / SSIM / 平均亮度差
tools/capture.mjs  对拍矩阵：壁纸 × 分辨率 × (正常, 藏 canvas)

实测：xilian 1920x1080 spine 层贡献 mean|diff|=26.1/255，kv37 为 10.2/255，
同配置两次截图噪声底 0.68/255。

tools/diff.mjs

@@ -0,0 +1,49 @@
+// 两张 PNG 的数值对拍：PSNR / SSIM / 平均亮度差（0-255）。
+// 用法：node tools/diff.mjs <a.png> <b.png> [diff-out.png]
+// 判读：平均差 < 1 视为肉眼无差异；1-3 为细微；> 8 通常意味着构图/内容变了。
+
+import { spawn } from "node:child_process";
+
+const [a, b, outPng] = process.argv.slice(2);
+if (!a || !b) {
+  console.error("usage: node tools/diff.mjs <a.png> <b.png> [diff-out.png]");
+  process.exit(2);
+}
+
+function ff(args) {
+  return new Promise((resolve) => {
+    const p = spawn("ffmpeg", ["-hide_banner", "-loglevel", "info", ...args], { stdio: ["ignore", "pipe", "pipe"] });
+    let err = "";
+    p.stderr.on("data", (d) => (err += d));
+    p.stdout.on("data", () => {});
+    p.on("close", () => resolve(err));
+  });
+}
+
+const psnr = await ff(["-i", a, "-i", b, "-lavfi", "psnr", "-f", "null", "-"]);
+const ssim = await ff(["-i", a, "-i", b, "-lavfi", "ssim", "-f", "null", "-"]);
+const yavg = await ff([
+  "-i", a, "-i", b,
+  "-lavfi", "blend=all_mode=difference,signalstats,metadata=print:key=lavfi.signalstats.YAVG",
+  "-f", "null", "-",
+]);
+
+const num = (text, re) => {
+  const m = text.match(re);
+  return m ? Number(m[1]) : NaN;
+};
+
+const yavgs = [...yavg.matchAll(/YAVG=([\d.]+)/g)].map((m) => Number(m[1]));
+
+const report = {
+  psnr_avg: /average:inf/.test(psnr) ? Infinity : num(psnr, /average:([\d.]+)/),
+  ssim_all: num(ssim, /All:([\d.]+)/),
+  mean_abs_diff: yavgs.length ? yavgs[yavgs.length - 1] : NaN,
+};
+
+console.log(`psnr=${report.psnr_avg} dB   ssim=${report.ssim_all}   mean|diff|=${report.mean_abs_diff}/255`);
+
+if (outPng) {
+  await ff(["-y", "-i", a, "-i", b, "-lavfi", "blend=all_mode=difference", "-frames:v", "1", outPng]);
+  console.log(`diff image -> ${outPng}`);
+}