The matched foreign draw implemented¶

holdout.matched_foreign_draw builds item 3 of the H2 plan: for one run's sibling draw it takes as many in-language rows from the foreign pool as the sibling holds and fills the rest from the pool's other rows, each stratum shuffled by the run's training seed and the sibling it is matched to (a refinement of item 3's "the run's training seed", so a run's two foreign adapters get independent draws). A pool that repeats an example id is refused, since repeats would count as capacity. A pool that cannot cover a draw is refused, never trained smaller, since that is a source that does not qualify. The partition jobs call it once a source's maintainers grant permission and its pool is built; no foreign draw has been made.