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.

changelog.d/1173-antimeridian-ground-floor.md

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+- Flights' ground floor works across the antimeridian. The viewer-distance
+  check behind the 150 km floor radius and its copy in the photoreal mesh-floor
+  sampler subtracted longitudes without wrapping, so two points 21 km apart
+  across 180° measured about 38,000 km and low or grounded aircraft just across
+  the line from the viewer were never floored. A contact's display corridor
+  walked a segment across 180° the long way round, spending its cell budget on
+  ground behind the contact, and a contact on the seam lost its cell hysteresis.
+  SEARCH NEARBY SITES on a view across 180° searched Google Places around
+  longitude 0, on the far side of the globe, with a radius sized from the view's
+  height alone; it now searches the view (Aditya, @Rohilalala, #1173).