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.
Add ValidateCorpus and CorpusStatsFrom for structural integrity checks
on the semantic route corpus. Validates total counts, route/source sums,
fast-path+residual partition, empty SourceID/SplitGroup, invalid routes,
duplicate identity, and conflicting labels on identical text.
CorpusStats provides deterministic dataset hash (SHA-256 of sorted texts,
first 16 bytes). ReproducibilityMeta in CorpusEnvelope records source
fixture hashes, generator version, split algorithm, and dataset hash.
TestCorpusValidation exercises the full validation pipeline.
claude