Start v2 safety hardening

eval: save adversarial results at both t=0.50 and t=0.85, reset production threshold to 0.50 (Day 14)

fix: threshold sweep — calibrate DeBERTa guardrail threshold, patch eval imports (Day 14)

feat: eval scripts — BM25 baseline, adversarial probes, BERTScore, RAGAS, Wilcoxon (Day 14)