没有直接撬 Step 1~9 的核心状态机，而是先把最适合独立的管理器和桥接层拆出来

tests/background-panel-bridge-module.test.js

@@ -0,0 +1,17 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+
+test('background imports panel bridge module', () => {
+  const source = fs.readFileSync('background.js', 'utf8');
+  assert.match(source, /importScripts\([\s\S]*'background\/panel-bridge\.js'/);
+});
+
+test('panel bridge module exposes a factory', () => {
+  const source = fs.readFileSync('background/panel-bridge.js', 'utf8');
+  const globalScope = {};
+
+  const api = new Function('self', `${source}; return self.MultiPageBackgroundPanelBridge;`)(globalScope);
+
+  assert.equal(typeof api?.createPanelBridge, 'function');
+});