Implement transportation emissions calculator module

Create a TypeScript module that estimates CO2 emissions for transportation. The module should export a function:

calculateTransportEmissions(input: { mode: TransportMode, distanceKm: number, passengers?: number }): { co2Kg: number, comparison: string }

Supported modes: car_gas, car_electric, car_hybrid, bus, train, subway, bicycle, walking, plane_domestic, plane_international

Emission factors should be based on EPA and DEFRA published data (hardcode reasonable defaults with source comments). The comparison field should return a human-readable string like "equivalent to charging 45 smartphones".

Include unit tests covering all transport modes and edge cases (0 distance, negative values).