fix(critical): Zt sign convention — align with Belkin & Suchower (1998)

BUG: Formula used (d - sqrt_rho*z) but correct is (d + sqrt_rho*z)
- Our thresholds are cumulative ascending (AAA→CCC→D)
- Higher Z should push probability mass left (better ratings)
- Previous: Z+ = higher PD = bad economy (WRONG)
- Fixed:    Z+ = lower PD = good economy (matches paper)

Verification:
- 1998 IMF crisis: Zt=-2.12 (negative = bad ✅)
- 2006 boom:       Zt=+1.53 (positive = good ✅)
- Pipeline 8/8 validation pass

models/credit_cycle.py

@@ -88,15 +88,17 @@ def model_transition_prob(
     sqrt_1_rho = np.sqrt(1.0 - rho)
     
     # 상한 임계값
+    # 논문: P(j|Z) = Φ((d_upper + √ρ·Z)/√(1-ρ)) - Φ((d_lower + √ρ·Z)/√(1-ρ))
+    # Z>0 (호황) → 누적확률 증가 → 상위등급 확률↑, 부도확률↓
     d_upper = thresholds[i, j]
-    upper = norm.cdf((d_upper - sqrt_rho * z) / sqrt_1_rho)
+    upper = norm.cdf((d_upper + sqrt_rho * z) / sqrt_1_rho)
     
     # 하한 임계값 (j=0이면 -∞)
     if j == 0:
         lower = 0.0
     else:
         d_lower = thresholds[i, j - 1]
-        lower = norm.cdf((d_lower - sqrt_rho * z) / sqrt_1_rho)
+        lower = norm.cdf((d_lower + sqrt_rho * z) / sqrt_1_rho)
     
     return max(upper - lower, 0.0)