Add an offline terrain shadow overlay to the Overlays panel

Shades the visible map by whether each point is currently blocked from
the sun by surrounding terrain (shademap.app-style), computed fully
offline via the same Terrarium DEM tiles the Terrain shading layer
already renders, plus this session's sun-position math and Radar's
checkbox+expandable UI shape.

dem.js pulls elevation-at-a-point out on its own since TODO.adoc's
slope readout and solar calculator will need the same lookup.

public/js/overlays.js

@@ -3,6 +3,7 @@ import { OVERLAYS_ICON, WIKI_ICON, WILDERNESS_ICON, MAP_MARKER_ICON } from "/js/
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import { svgToImageData, badgeImageId, escapeHtml } from "/js/gpx.js";
 import { RadarControl } from "/js/radar.js";
+import { ShadowControl } from "/js/shadow.js";
 
 const ICON_IMAGE_SIZE = 48; // rasterized once at this size, rendered much smaller by icon-size below
 const ICON_SIZE = 0.3; // 48 * 0.3 = ~14px, sized to sit inside the marker badge below
@@ -35,7 +36,9 @@ const ELEVATION_COLOR_RAMP = [
 // is skipped in favor of a local terrain.pmtiles if one's been generated
 // (see docs/development.adoc), same optional-local-file story as
 // offline.pmtiles itself.
-const ONLINE_TERRAIN_TILES = ["https://s3.amazonaws.com/elevation-tiles-prod/terrarium/{z}/{x}/{y}.png"];
+// Exported: dem.js's elevation lookups (shadow.js's raymarch) hit the same
+// online tile source, rather than duplicating the URL template.
+export const ONLINE_TERRAIN_TILES = ["https://s3.amazonaws.com/elevation-tiles-prod/terrarium/{z}/{x}/{y}.png"];
 
 // A plain on/off Overlays item — there's nothing to choose, just whether
 // the shading shows, unlike Radar's expandable scrub/forecast.
@@ -121,6 +124,7 @@ export class OverlaysControl {
     this._map = map;
     this._hillshade = new HillshadeControl(map);
     this._radar = new RadarControl(map);
+    this._shadow = new ShadowControl(map);
     this._wikiEnabled = false; // the checkbox's own state
     this._wikiMoveHandler = null; // the moveend listener driving its live refetch, while enabled
     this._wikiRequestSeq = 0; // ignores an in-flight refetch a newer one has already superseded
@@ -152,11 +156,13 @@ export class OverlaysControl {
       <li class="overlays-panel__item"></li>
       <li class="overlays-panel__item"></li>
       <li class="overlays-panel__item overlays-panel__item--expandable"></li>
+      <li class="overlays-panel__item overlays-panel__item--expandable"></li>
     `;
-    const [hillshadeItem, wikiItem, radarItem] = list.children;
+    const [hillshadeItem, wikiItem, radarItem, shadowItem] = list.children;
     this._hillshade.mount(hillshadeItem);
     this._mountWiki(wikiItem);
     this._radar.mount(radarItem);
+    this._shadow.mount(shadowItem);
 
     onlyOneSidePanelOpen(this._panel, () => this._icon.classList.remove("active"));
 
@@ -174,6 +180,10 @@ export class OverlaysControl {
     this._hillshade.setOfflineAvailable(available);
   }
 
+  setShadowOfflineAvailable(available) {
+    this._shadow.setOfflineAvailable(available);
+  }
+
   _mountWiki(li) {
     li.innerHTML = `
       <label>