Add a zoom-level readout to the map control stack

Matches the small numbered square NPS keeps next to its own zoom
buttons — a passive display, not a button, so it's plain markup with
no click behavior.

docs/architecture.adoc

@@ -145,8 +145,11 @@ app's default view, not the visitor's own location — that's
 
 That Home button is the top of a small top-left stack — Home, zoom
 in/out (`NavigationControl`, `showCompass: false` since this map never
-rotates), locate-me (`GeolocateControl`), and a "Base map" button —
-matching the National Park Service's own map control layout (grouped
+rotates), a passive zoom-level readout (`ZoomLevelControl`, a plain div
+updated on the map's `zoom` event — not a button, since it has no
+click behavior of its own), locate-me (`GeolocateControl`), and a
+"Base map" button — matching the National Park Service's own map
+control layout (grouped
 together on the map itself, rather than split across corners or
 between the map and a side panel). The last of those,
 `MapLayersButton` (`public/js/map.js`), is a second entry point into