- fetch_gtfs() in remote_data.py: auto-discovers feeds via Mobility
  Database catalog (ranked by IoU), parses GTFS ZIP (shapes, stops,
  routes), supports direct URL and local file inputs
- place_gtfs() in accessor.py: groups routes by route_color so each
  transit line renders in its official color (e.g. MTA subway trunk
  lines), with per-mode width multipliers and stop markers
- GTFS-RT realtime overlay in engine.py: background poll thread for
  protobuf vehicle positions, colored dots projected to screen space,
  toggled with Shift+B
- NaN guard in geojson.py _densify_on_terrain for edge-of-raster coords
- Added gtfs to NYC example features list