Register Rio's two keyless GTFS-Realtime VehiclePositions feeds from the
city transport secretariat (SMTR): municipal buses (SPPO) and the
Mobi-Rio BRT, the Transit layer's first Latin American region.
Both feeds publish Position.speed in km/h instead of m/s. A new optional
registry field, speedUnit: 'kmh', makes buildTransitSnapshot convert it
once, so every speedMps consumer stays correct and other feeds are
untouched.
Document the source in DATA_SOURCES.md and update the region lists in
README.md and docs/CURRENT-STATE.md.
Refs #1480
Co-Authored-By: Claude Opus 5.5 <[email protected]>
