feat: Phase U U2 batch release checklist (BRC-) -- machine-verifiable wet-lab synthesis gate

[cschanhniem]

Summary

• Adds BRC- (Batch Release Checklist) schema: machine-verifiable gate before wet-lab synthesis
• Gates on 3 checklist items: PCC grade ≥ B, BEG verdict not critical_gaps/no_families, SAT entries ≥ 1
• VALID_PCC_GRADES (A/B/C/D), PASSING_PCC_GRADES (A/B), BLOCKING_BEG_VERDICTS (critical_gaps/no_families)
• VALID_RELEASE_VERDICTS: approved/blocked/conditional
• build() auto-computes 3 ChecklistItem records with pass/fail details
• dry_lab_only=True enforced; BRC-/PCC-/BEG-/SAT- prefixes validated
• 60 tests; BASELINE 9891→9954

Test plan

• 60 unit tests pass: constants, build happy paths, validate rejections, format output
• Regression test passes
• Smoke test: approved on all-pass, blocked on pcc_grade=D or critical_gaps or n_sat_entries=0