Share one antimeridian-safe distance and move the changelog entry

The flights records and the mesh-floor sampler kept identical copies of the equirectangular distance; both now import src/data/approxDistance.js. The changelog entry moves to changelog.d (#787), and CURRENT-STATE notes the antimeridian behaviour.

src/data/approxDistance.js

@@ -0,0 +1,12 @@
+/**
+ * Local equirectangular distance in km, for viewer-proximity gates. Takes the
+ * short way round, so points either side of 180° stay close.
+ */
+export function approxDistanceKm(lat1, lon1, lat2, lon2) {
+  const dLat = (lat2 - lat1) * 111.32;
+  let dLonDeg = lon2 - lon1;
+  dLonDeg -= 360 * Math.round(dLonDeg / 360);
+  const dLon =
+    dLonDeg * 111.32 * Math.cos((((lat1 + lat2) / 2) * Math.PI) / 180);
+  return Math.hypot(dLat, dLon);
+}