Function export_dfjson 

pub fn export_dfjson(content: &[u8], opts: &DfjsonOptions) -> String

Export the IfcSpace volumes in content (raw IFC/STEP bytes) as a Dragonfly DFJSON string. Each space becomes an extruded Room2D (floor polygon + heights) grouped into stories — the simpler Ladybug target for mostly-vertical-wall models.