feat: Phase W W4 benchmark challenge registry schema (BCR-)

[cschanhniem]

Summary

• Adds BCR- (benchmark challenge registry) schema: machine-readable registry of NCH/CMC/SCH challenge results for a batch+pipeline version
• Maps raw challenge verdicts to pass/marginal/fail/not_run (e.g. novel_batch→pass, mixed_novelty→marginal, near_neighbor_dominated→fail)
• Hardness grade: A (all pass), B (all pass+marginal), C (some pass/marginal), D (all fail/not_run)
• ChallengeEntry helper stores challenge_type, artifact_id, raw_verdict, challenge_verdict
• 64 tests covering constants, build happy paths, verdict mapping, grade computation, validate rejection, format

Test plan

• python3 -m pytest tests/evidence/test_benchmark_challenge_registry.py -- 64 passed