feat(app): construct the globe engine with the viewer

The scene builds the page's Cesium engine right after the viewer
(src/app/globeEngine.js), names the startup Google tileset to it once
loaded, returns it beside the viewer and destroys it before the viewer.
The debug handle exposes it as window.__godsEyeView.engine. Nothing
consumes it yet; render holds taken before the governor is installed
apply at install.

qa-globe-contract now runs on the app's engine (self-built only when the
app has none); a throwaway engine checks creation and destroy leave the
scene untouched.

src/app/application.test.mjs

@@ -83,6 +83,32 @@ test('startup shares one promise and teardown keeps data alive for controls', as
   await assert.rejects(app.start(), /destroyed/);
 });
 
+test('later components receive the scene engine beside its viewer', async () => {
+  const viewer = { name: 'viewer' };
+  const engine = { kind: 'cesium' };
+  const seen = [];
+  const receive =
+    (phase) =>
+    ({ scene }) => {
+      seen.push([phase, scene.viewer, scene.engine]);
+      return { name: phase };
+    };
+  const { app } = fixture({
+    createScene: () => ({ viewer, engine }),
+    createControls: receive('Controls'),
+    createData: receive('Data'),
+    createTools: receive('Tools'),
+  });
+  const components = await app.start();
+  assert.equal(components.scene.engine, engine);
+  assert.deepEqual(seen, [
+    ['Controls', viewer, engine],
+    ['Data', viewer, engine],
+    ['Tools', viewer, engine],
+  ]);
+  await app.destroy();
+});
+
 for (const phase of phases) {
   test(`failure during ${phase} releases partially acquired resources`, async () => {
     const { app, events } = fixture({