…ture model (#31)

- ➕ Implement GeoJsonTrack class supporting LineString and FeatureCollection formats
- 📊 Add cum_time_stopped column to track stopped duration separately from moving time
- 🔧 Refactor cumulative time calculation to independently track moving vs stopped periods
- 🛡️ Handle edge case where all points are stationary by interpreting as moving
- 📝 Add comprehensive GeoJSON schema validation with Pydantic discriminated unions

- Add `GeoJsonTrack` to public API exports in `__init__.py`
- Extend `GeoJsonTrack.__init__()` to accept `dict` type for direct GeoJSON data
- Reorder source type checking to handle dict before string/bytes