refactor: separate application UI state owners

src/data/layerState.test.mjs

@@ -1,3 +1,4 @@
+import { readShellSource } from '../testSupport/readShellSource.mjs';
 import { expandApplicationHtml } from '../../build/application-html.js';
 import { readLayerSource } from '../testSupport/readLayerSource.mjs';
 import test from 'node:test';
@@ -373,7 +374,7 @@ test('a fresh boot starts 3D aircraft ON in proximity — codec, both layers, an
     assert.match(source, /^\s*(?:let |flightState\.)_models3dMode = 'proximity';/m,
       `${name}: and starts in proximity, matching the codec default`);
   }
-  const ui = await readFile(new URL('../ui/applicationShell.js', import.meta.url), 'utf8');
+  const ui = await readShellSource();
   assert.match(ui, /^\s*this\.(?:flightState\.)?_models3dEnabled = true;$/m,
     'ui.js: the DISPLAY rail believes 3D is on before any layer-state sync arrives');
   assert.match(ui, /this\.(?:flightState\.)?_models3dMode = 'proximity';/,