docs(changelog): add fragment for #1481

Co-Authored-By: Claude Opus 5.5 <[email protected]>

changelog.d/1481-rio-transit.md

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+- Add Rio de Janeiro to the Transit layer: municipal buses and the Mobi-Rio BRT
+  from SMTR's keyless GTFS-Realtime feeds (CC BY 4.0), the layer's first Latin
+  American region. A feed that publishes speed in km/h declares
+  `speedUnit: 'kmh'` and the proxy converts it to m/s (IMBrandao, #1481).