Function run_r 

pub fn run_r(columns: &[Column<'_>], body: &str) -> ReferenceResult

Run an R reference body. The columns are exposed as a data.frame named df; the body calls emit("key", numeric_vector) to return results. The harness prepends the df, output path, and emit helper. Fails the test with the captured stderr when R exits non-zero — a broken or unavailable reference run (missing Rscript, missing package, R error) is a hard test failure, never a silent skip.