Function build_retrieval_area 

pub fn build_retrieval_area(
    positions: &[GnssPosition],
    buffer_meters: f64,
) -> Result<RetrievalArea, ProjectionError>

Build a 1 km-expanded (configurable) WGS84 axis-aligned search polygon from GNSS positions.

Returns Err(InvalidGnssInput) if no usable WGS84 coordinates are present.