refactor(geo): define shared distance contract

src/locations.js

@@ -1,6 +1,7 @@
 import { createOverpassFeatureSource } from './sources/overpassFeatures.js';
 import { applicationServices } from './services/application.js';
 import * as Cesium from 'cesium';
+import { greatCircleKm } from './geoDistance.js';
 import {
   viewportBias,
   placesNearViewRecovery,
@@ -1103,18 +1104,6 @@ export const PLACE_ANCHOR_OFFSET_RATIO = 0.15;
  */
 const PLACE_FALLBACK_HALF_SPAN_KM = 20;
 
-/** Great-circle distance in km (small enough here that the spherical model is fine). */
-function greatCircleKm(lat1, lng1, lat2, lng2) {
-  const dLat = Cesium.Math.toRadians(lat2 - lat1);
-  const dLng = Cesium.Math.toRadians(lng2 - lng1);
-  const a =
-    Math.sin(dLat / 2) ** 2 +
-    Math.cos(Cesium.Math.toRadians(lat1)) *
-      Math.cos(Cesium.Math.toRadians(lat2)) *
-      Math.sin(dLng / 2) ** 2;
-  return 6371 * 2 * Math.asin(Math.min(1, Math.sqrt(a)));
-}
-
 /**
  * Sanity-gate a geocode viewport before framing it. Pure — exported for unit tests.
  *
@@ -1149,10 +1138,8 @@ export function placeFramingViewport(
     return viewport;
 
   const offsetKm = greatCircleKm(
-    anchorLat,
-    anchorLng,
-    metrics.centerLat,
-    metrics.centerLng,
+    { lat: anchorLat, lon: anchorLng },
+    { lat: metrics.centerLat, lon: metrics.centerLng },
   );
   if (offsetKm <= metrics.spanKm * PLACE_ANCHOR_OFFSET_RATIO) return viewport;