Add surface chemistry support to Cantera YAML export

- Segregate species and reactions into gas-phase and surface-phase lists
- Add 'ideal-surface' phase definition with site-density when surface
  species are present
- Support SurfaceArrhenius and StickingCoefficient kinetics types
- Serialize coverage dependencies to Cantera YAML format
- Pass site_density from RMG settings through to generate_cantera_data()
- Skip transport data for surface species
- Exclude surface site marker 'X' from species compositions
- Extract get_reaction_equation() helper from reaction_to_dict_list()
- Remove redundant MultiArrhenius branch (already handled at top of function)
- Clean up comments and minor formatting

Author: alongd