Function write_team_composition_csv 

pub fn write_team_composition_csv<W: Write>(
    rows: &[TeamCompositionRow],
    w: &mut W,
) -> Result<()>

team-composition CSV emitter.

CSV header: author,tenure-days,bucket,veteran-breadth-ok,active,commits,files-touched,onboarding-weeks

The onboarding-weeks column is empty for NULL (founders and authors who never reached the weekly 80%-core set). The __summary__ carrier row (bucket-share percentages, not per-author data) is not a data row and is skipped.