add official Chrome feature demos

public/demo/shared.js

@@ -0,0 +1,24 @@
+export function logTo(target, title, payload) {
+  const el = typeof target === "string" ? document.querySelector(target) : target;
+  if (!el) return;
+  const stamp = new Date().toLocaleTimeString("zh-CN", { hour12: false });
+  const entry = {
+    time: stamp,
+    event: title,
+    ...payload,
+  };
+  el.textContent = `${JSON.stringify(entry, null, 2)}\n\n${el.textContent}`.trim();
+}
+
+export function setText(selector, value) {
+  const el = document.querySelector(selector);
+  if (el) el.textContent = value;
+}
+
+export function escapeHtml(value) {
+  return String(value)
+    .replaceAll("&", "&")
+    .replaceAll("<", "&lt;")
+    .replaceAll(">", "&gt;")
+    .replaceAll('"', "&quot;");
+}