Function default_version 

fn default_version() -> u32

Default checkpoint version for backward compatibility.

Version history:

• Version 1: Legacy format with coordinates in Angstrom (original)
• Version 2: Legacy format with coordinates in Bohr (intermediate)
• Version 3: Current format with coordinates in Angstrom (unit standardization)

The default is version 1 for backward compatibility when loading old checkpoints that don’t have a version field.