Add OpenSeaMap / OpenSnowMap overlay layers (#10)

Two independently toggleable open-data raster overlays — OpenSeaMap
sea marks (buoys, lighthouses, ports) and OpenSnowMap ski pistes —
drawn as Cesium.ImageryLayers directly on the Cesium globe surface.
Same "small array of descriptors" shape as MAP_STACKS in
mapStackController.js.

Visible on the osm/bing-aerial/bing-labels map stacks and invisible
(not broken — the layer's stats say so in plain text) on the default
photoreal stack, since Google Photorealistic 3D Tiles render with the
globe hidden.

OpenRailwayMap and OpenInfraMap, also requested in #10, are
deliberately NOT included in this pass: OpenRailwayMap's public tile
server enforces a Referer allowlist scoped to its own site (confirmed
by direct request — every other Referer, including none, 403s), and
OpenInfraMap serves MapLibre GL vector tiles, not raster XYZ — that
would need a genuine vector-tile rendering pipeline, not a
Cesium.ImageryLayer drop-in. Worth its own dedicated pass.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/standalone/data.js

@@ -10,6 +10,7 @@ import radioLayer from '../data/radio.js';
 import bikeshareLayer from '../data/bikeshare.js';
 import aisLiveVesselsLayer from '../data/aisLiveVessels.js';
 import militaryInstallationsLayer from '../data/militaryInstallations.js';
+import osmOverlaysLayer from '../data/osmOverlays.js';
 import militaryAwarenessLayer from '../data/militaryAwareness.js';
 import localDataLayers from '../data/localLayers.js';
 import { LAYER_STATE_REGISTRY } from '../data/layerState.js';
@@ -44,6 +45,7 @@ export function createStandaloneData({
   dataManager.register(bikeshareLayer);
   dataManager.register(aisLiveVesselsLayer);
   dataManager.register(militaryInstallationsLayer);
+  dataManager.register(osmOverlaysLayer);
   dataManager.register(militaryAwarenessLayer);
   militaryAwarenessLayer.attachDataManager(dataManager);
   for (const layer of localDataLayers) {