feat(transit): add Rio de Janeiro buses and BRT (SMTR GTFS-RT, CC BY 4.0)

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]>

docs/CURRENT-STATE.md

@@ -4295,11 +4295,13 @@ Transit is off by default, and share links use token `j`. The movement panel ord
 places Transit directly after Street Traffic and before Bike Share. Cameras and
 utilities retain their own groups. The layer polls
 registered feeds near the camera every 15 seconds, below a 3,000 km altitude
-gate. Sixteen keyless regions are available: Boston, Austin, Minneapolis–St Paul,
-Helsinki, the Netherlands, Norway, South East Queensland, and nine in
+gate. Seventeen keyless regions are available: Boston, Austin, Minneapolis–St Paul,
+Helsinki, the Netherlands, Norway, South East Queensland, Rio de Janeiro
+(municipal buses and the Mobi-Rio BRT, two feeds), and nine in
 Ontario — Toronto (TTC surface vehicles; the subway publishes no
 positions), MiWay, Hamilton HSR, Durham Region, YRT/Viva, Guelph,
-Burlington, Barrie and Kingston. Mode silhouettes
+Burlington, Barrie and Kingston. Rio's feeds publish speed in km/h, which
+the proxy converts to m/s before any card reads it. Mode silhouettes
 identify buses, trams, subways, trains, ferries and unclassified vehicles.
 
 Retained transit history uses the same outlier quarantine as live observations and