Add FrozenHoldoutSplit with deterministic 15% ratio using dedicated hash
seed. Produces frozen/dev partition with SplitGroup-aware leakage
prevention — all contrastive variants of one seed stay in the same split.
Add GroupedCVFolds for k-fold grouped cross-validation on the development
pool. Each fold preserves split_group boundaries; every example appears
in exactly one eval set across all folds.
Tests verify determinism (same split → same hash), no split_group leakage
across frozen/dev, route coverage in both pools, fold completeness, and
grouped CV coverage.
claude
d63619bd0e