Embed the files Cesium's workers load in the panel's workers script

The ground-polyline worker loads approximateTerrainHeights.json itself,
and a worker's requests go to the panel page's own site, where they
failed with 404 and stopped rendering once ground-clamped lines such as
military installations appeared. The panel build now embeds that file in
the workers script, which answers the worker's request from memory.

src/tooling/panelBuild.test.mjs

@@ -9,6 +9,7 @@ import {
   PANEL_WORKERS_PATH,
   panelBuildConfig,
   panelBuildPlugin,
+  workerFilesPrelude,
   workersEntrySource,
 } from '../../build/panel.js';
 
@@ -81,3 +82,46 @@ test('the dev server serves the panel build and nothing outside it', async () =>
     await rm(root, { recursive: true, force: true });
   }
 });
+
+test("Cesium's workers get embedded files from memory and other requests from the network", async () => {
+  const sent = [];
+  class FakeRequest extends EventTarget {
+    open(method, url) {
+      this.url = url;
+    }
+    setRequestHeader() {}
+    send() {
+      sent.push(this.url);
+    }
+  }
+  const saved = {
+    XMLHttpRequest: globalThis.XMLHttpRequest,
+    ProgressEvent: globalThis.ProgressEvent,
+  };
+  globalThis.XMLHttpRequest = FakeRequest;
+  globalThis.ProgressEvent ??= class extends Event {};
+  try {
+    new Function(workerFilesPrelude({ 'Assets/heights.json': '{"a":1}' }))();
+    const embedded = new XMLHttpRequest();
+    embedded.open(
+      'GET',
+      'https://panel.example/panel/cesium/Assets/heights.json?v=1',
+    );
+    embedded.setRequestHeader('Accept', 'application/json');
+    embedded.responseType = 'text';
+    const loaded = new Promise((resolve) =>
+      embedded.addEventListener('load', resolve),
+    );
+    embedded.send();
+    await loaded;
+    assert.equal(embedded.status, 200);
+    assert.equal(embedded.response, '{"a":1}');
+    const other = new XMLHttpRequest();
+    other.open('GET', 'https://tiles.example/1.png');
+    other.send();
+    assert.deepEqual(sent, ['https://tiles.example/1.png']);
+  } finally {
+    globalThis.XMLHttpRequest = saved.XMLHttpRequest;
+    globalThis.ProgressEvent = saved.ProgressEvent;
+  }
+});