B3: Add baseline_caveat field to evidence certificates

[cschanhniem]

Bottleneck

Certificates do not document which cheap baselines the scores beat or failed to beat. Reviewers cannot assess shortcut risk.

Task

Add baseline_caveat field to candidate.schema.json. Document which baselines exist for each score (charge, length, hydrophobicity) and whether each score outperforms them.

Acceptance criteria

• Schema accepts baseline comparison records
• Generator includes baseline comparison from available benchmark results
• 2+ tests

Source

NEXT_100_PR_MAP, B3

[cschanhniem]

Already completed in prior loops.