feat: Phase W W2 charge-matched challenge schema (CMC-)

[cschanhniem]

Summary

• Adds CMC- (charge-matched challenge) schema: formally documents the pipeline AUROC vs charge-only baseline comparison
• Verdict auto-computed: gap_meaningful (≥0.05), gap_marginal (0.02–0.05), gap_absent (<0.02), challenge_not_run (n=0)
• 4 baseline methods (charge_only_rank, charge_length_rank, charge_hydrophobicity_rank, logistic_charge_only)
• AUROC gap auto-computed from pipeline_auroc - baseline_auroc; validated against stored value
• Blocks performance claims when charge-only baseline explains the selection signal
• dry_lab_only=True enforced; 60 tests

Test plan

• python3 -m pytest tests/evidence/test_charge_matched_challenge.py -- 60 passed