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.
136 examples with provenance from ru_routing_v1.json, tagged
fast_path_resolved vs residual. RouteExample carries source, source_id,
split_group for traceability. Split-by-family prevents paraphrase
leakage across train/eval.
claude