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/data/layerState.test.mjs

@@ -11,7 +11,6 @@ import {
   LAYER_STATE_STORAGE_KEY,
   LAYER_STATE_TOKEN_ALPHABET,
   LAYER_STATE_TOKEN_RESERVATIONS,
-  LayerStateCoordinator,
   REGISTERED_LAYER_IDS,
   SHARE_TRACKING_RESTORE_POLICIES,
   createDefaultLayerState,
@@ -24,6 +23,7 @@ import {
   validateLayerStateAllocations,
   validateLayerStateRegistry,
 } from './layerState.js';
+import { LayerStateCoordinator } from './layerStateCoordinator.js';
 import radioLayer from './radio.js';
 import { stampInitialShareGesture } from '../navigationPolicy.js';