refactor(geo): define shared distance contract

src/data/trafficBounds.js

@@ -13,31 +13,11 @@
  * @module data/trafficBounds
  */
 
-/** @const {number} Mean Earth radius in km (spherical approximation). */
-const EARTH_RADIUS_KM = 6371;
+import { EARTH_RADIUS_KM, greatCircleKm, isGeoPoint } from '../geoDistance.js';
 
 const toRad = (deg) => (deg * Math.PI) / 180;
 const toDeg = (rad) => (rad * 180) / Math.PI;
 
-/**
- * Great-circle distance between two lat/lon points (haversine).
- *
- * @param {number} lat1 - First point latitude (degrees).
- * @param {number} lon1 - First point longitude (degrees).
- * @param {number} lat2 - Second point latitude (degrees).
- * @param {number} lon2 - Second point longitude (degrees).
- * @returns {number} Distance in kilometres.
- */
-export function greatCircleKm(lat1, lon1, lat2, lon2) {
-  const p1 = toRad(lat1);
-  const p2 = toRad(lat2);
-  const dp = toRad(lat2 - lat1);
-  const dl = toRad(lon2 - lon1);
-  const a =
-    Math.sin(dp / 2) ** 2 + Math.cos(p1) * Math.cos(p2) * Math.sin(dl / 2) ** 2;
-  return 2 * EARTH_RADIUS_KM * Math.asin(Math.min(1, Math.sqrt(a)));
-}
-
 /**
  * Initial bearing (radians) from point 1 toward point 2 along the great circle.
  *
@@ -111,10 +91,13 @@ export function deriveFetchCenter({
   hitLon,
   maxPullKm = 12,
 }) {
-  if (!Number.isFinite(hitLat) || !Number.isFinite(hitLon)) {
+  if (!isGeoPoint({ lat: hitLat, lon: hitLon })) {
     return { lat: nadirLat, lon: nadirLon, source: 'nadir' };
   }
-  const distKm = greatCircleKm(nadirLat, nadirLon, hitLat, hitLon);
+  const distKm = greatCircleKm(
+    { lat: nadirLat, lon: nadirLon },
+    { lat: hitLat, lon: hitLon },
+  );
   if (distKm <= maxPullKm) {
     return { lat: hitLat, lon: hitLon, source: 'hit' };
   }