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/data/meshFloorSampler.test.mjs

@@ -128,3 +128,12 @@ test('sampler: outside the google-3d regime nothing is sampled at all', () => {
   setMeshFloorPreferred(true);
   assert.equal(cachedMeshFloor(cell.lat, cell.lon), null);
 });
+
+test('sampler: a cell just across the antimeridian is still near the viewer', () => {
+  reset();
+  let calls = 0;
+  const scene = fakeScene(() => { calls += 1; return undefined; });
+  sampleMeshFloorCells(scene, [{ lat: -16.8, lon: -179.999 }], { viewerLat: -16.8, viewerLon: 179.999 });
+  sampleMeshFloorCells(scene, [{ lat: -16.81, lon: 179.999 }], { viewerLat: -16.81, viewerLon: -179.999 });
+  assert.equal(calls, 2, 'about 200 m apart, so both cells are probed');
+});