fix(geo): wrap longitude differences across the antimeridian

The flights ground-floor pipeline subtracted longitudes without wrapping.
approxDistanceKm, and its copy in the mesh-floor sampler, measured two
points 0.2° apart either side of 180° (21 km at Fiji) as about 38,000 km,
so low and grounded aircraft just across the line from the viewer were
never floored or mesh-sampled. corridorFloorCells walked a segment across
180° the long way round, filling its cell budget with ground behind the
contact and never requesting the cells it was about to cross, and
stickyFloorCell dropped its hysteresis for a contact on the seam. Each now
takes the short way round, and corridor points are normalised back into
[-180, 180].

SEARCH NEARBY SITES centred its Google Places search on the mean of west
and east, which for a view across 180° (west > east) is longitude 0, and
sized the radius from the view's height alone. It now measures the view's
width through 180° and searches its real centre.

src/layers/flights/recordPolicy.js

@@ -15,7 +15,10 @@ export const ERROR_BACKOFF_INTERVAL = 20000;
 /** Local equirectangular distance used for viewer-proximate floor sampling. */
 export function approxDistanceKm(lat1, lon1, lat2, lon2) {
   const dLat = (lat2 - lat1) * 111.32;
+  // The short way round, so points either side of 180° stay close.
+  let dLonDeg = lon2 - lon1;
+  dLonDeg -= 360 * Math.round(dLonDeg / 360);
   const dLon =
-    (lon2 - lon1) * 111.32 * Math.cos((((lat1 + lat2) / 2) * Math.PI) / 180);
+    dLonDeg * 111.32 * Math.cos((((lat1 + lat2) / 2) * Math.PI) / 180);
   return Math.hypot(dLat, dLon);
 }

src/layers/flights/recordPolicy.test.mjs

@@ -0,0 +1,18 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  approxDistanceKm,
+  GROUND_FLOOR_CLAMP_RADIUS_KM,
+} from './recordPolicy.js';
+
+test('approximate distance takes the short way across the antimeridian', () => {
+  // 0.2° of longitude at Taveuni, Fiji (16.8° S) is about 21 km.
+  for (const [lon1, lon2] of [
+    [179.9, -179.9],
+    [-179.9, 179.9],
+  ]) {
+    const km = approxDistanceKm(-16.8, lon1, -16.8, lon2);
+    assert.ok(Math.abs(km - 21.31) < 0.01, `${lon1} → ${lon2}: ${km} km`);
+    assert.ok(km <= GROUND_FLOOR_CLAMP_RADIUS_KM);
+  }
+});