SparrowXL refresh kit (optional, power users)

The trial add-in refreshes EIA data by itself (eia-import.exe,
no Python). This kit adds:
- vx.exe: converts datasets to the Vortex format (faster reads;
  place beside the XLL and imports write .vortex automatically)
- eia_to_dataset.py: the Python twin of the importer, for
  scripted/scheduled refreshes (pip install polars pyarrow)

Both write the same digest_v2 manifest - the two importers can
share a folder without stepping on each other.
