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