docs(changelog): long-route corridor (#1655)

changelog.d/1655-long-route-corridor.md

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+- A long flight keeps its origin and destination when weather or airways take
+  it far off the great circle: the route check now allows 200 km or 15% of the
+  route's length, whichever is larger (623 km for JFK–SFO), while a short hop
+  still allows 200 km (Sameh Khamis, @samehkhamis, #1655).