feat: Phase W W1 novelty challenge harness schema (NCH-)

[cschanhniem]

Summary

• Adds NCH- (novelty challenge harness) schema: batch-level gate documenting what fraction of top candidates have ≥80% sequence identity to known AMPs in a reference database
• Verdict auto-computed: novel_batch (≤20% near-neighbors), mixed_novelty (20-60%), near_neighbor_dominated (≥60%), challenge_not_run (empty input)
• 6 reference databases (APD3, DRAMP, DBAASP, CAMP, LAMP, custom); configurable identity threshold
• NCHCandidateResult helper stores per-candidate identity and near-neighbor flag
• Blocks "novel family" batch-level overclaims without machine-verifiable evidence
• Adds Phase W definition (W1-W5) to NEXT_100_PR_MAP: batch-level benchmark hardness phase
• dry_lab_only=True enforced; empty limitations rejected; 63 tests

Test plan

• python3 -m pytest tests/evidence/test_novelty_challenge_harness.py -- 63 passed
• Regression BASELINE=9830, tolerance=491; 63 new tests within tolerance