Function render_unix 

pub fn render_unix(
    errors: &[LisetteDiagnostic],
    lints: &[LisetteDiagnostic],
    get_source: impl Fn(u32) -> Option<(String, String)>,
    file_count: usize,
    filter: &Filter,
    default_source: &str,
    default_filename: &str,
) -> (String, Counts)

Builds the stdout text (one diagnostic per line, no color, no banner) and the counts the caller needs for the stderr summary and exit code.