feat: Phase W W3 similarity challenge harness schema (SCH-)

[cschanhniem]

Summary

• Adds SCH- (similarity challenge harness) schema: checks whether pipeline-selected candidates are systematically more similar to known AMPs than a random draw from the same sequence space
• Verdict: selection_adds_value (gap≥0.10), marginal_improvement (0.03–0.10), proximity_driven (<0.03), challenge_not_run (n=0)
• SimilarityGroupStats helper stores group label, mean similarity to known AMPs, and n_sequences
• similarity_gap auto-computed and validated (pipeline_mean - random_mean)
• 4 similarity metrics (sequence_identity, blosum62_score, edit_distance, physicochemical_distance)
• Prevents "novel panel" claim when selection is proximity-driven
• dry_lab_only=True enforced; 60 tests

Test plan

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