Add the changelog fragment and document the shared distance contract

changelog.d/287-shared-great-circle-distance.md

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+- The client's great-circle distances now come from one shared
+  `src/geoDistance.js` (`greatCircleKm`/`greatCircleMeters`, `{lat, lon}`
+  points). The Earth radius is standardized on 6371.0088 km, so distances shift
+  by about 1.4 ppm and region areas by about 2.8 ppm, and missing or
+  out-of-range points return `Infinity` (onionviolet, #287; Rajojha-1, #749).