refactor: move the layer state coordinator into its own module

layerState.js held both the share-link layer codec and the coordinator
that restores layer choices into the running app and persists them to
browser storage. Move the coordinator, unchanged, to
layerStateCoordinator.js so the codec stays a pure data module that code
outside the browser can use to write share links. The registry stays in
layerState.js, where the token tooling reads it, and the aircraft
barrel re-exports both.

src/ui/shareRestoration.js

@@ -1,4 +1,4 @@
-import { LayerStateCoordinator } from '../data/layerState.js';
+import { LayerStateCoordinator } from '../data/layerStateCoordinator.js';
 import { stampInitialShareGesture } from '../navigationPolicy.js';
 import { canPresentDeferredStatusNotice } from '../loadingFeedback.js';
 import { UiLifetime } from './uiLifetime.js';