Three tasks: (1) extract the disjoint-parallel core out of sweep_with_threads
into a shared `pub(crate) run_indexed<T>` (behaviour-preserving; the three
pre-existing sweep tests are the green guard), (2) add the `mc.rs` module
(McDraw/McFamily/McAggregate/MetricStats, type-7 `quantile`,
`McAggregate::from_draws`, `monte_carlo`/`monte_carlo_with_threads` driving
run_indexed) with 7 RED tests, (3) wire `mod mc;` + the five public exports into
lib.rs. RED-first; module-declaration ordering keeps mc.rs's tests dark until the
final wire-up task.
refs #68
Brummel
5181a7132c