refactor(geo): define shared distance contract

src/data/naturalEarthRegions.js

@@ -17,8 +17,8 @@
  */
 
 import { createRetryableLoader } from './retryableLoad.js';
+import { EARTH_RADIUS_KM, greatCircleKm } from '../geoDistance.js';
 
-const EARTH_RADIUS_KM = 6371;
 const toRad = (d) => (d * Math.PI) / 180;
 
 /** Spherical-excess ring area (km²) — same family as turf/geojson-area. */
@@ -35,15 +35,6 @@ function ringAreaKm2(ring) {
   return Math.abs((sum * EARTH_RADIUS_KM * EARTH_RADIUS_KM) / 2);
 }
 
-function haversineKm(lon1, lat1, lon2, lat2) {
-  const dLat = toRad(lat2 - lat1);
-  const dLon = toRad(lon2 - lon1);
-  const h =
-    Math.sin(dLat / 2) ** 2 +
-    Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLon / 2) ** 2;
-  return 2 * EARTH_RADIUS_KM * Math.asin(Math.min(1, Math.sqrt(h)));
-}
-
 /**
  * Normalize a query/name for matching: lowercase, strip diacritics and
  * punctuation, collapse whitespace, strip a leading "the ".
@@ -162,7 +153,10 @@ function buildEntries(pack, kind) {
       polygons,
       areaKm2,
       bbox: [minLon, minLat, maxLon, maxLat],
-      bboxDiagonalKm: haversineKm(minLon, minLat, maxLon, maxLat),
+      bboxDiagonalKm: greatCircleKm(
+        { lat: minLat, lon: minLon },
+        { lat: maxLat, lon: maxLon },
+      ),
     });
   }
   return out;