fix(critical): complete Zt sign alignment across all modules

Fixed ALL instances of (d - sqrt_rho*z) -> (d + sqrt_rho*z):
- models/vasicek.py: conditional_transition_matrix() (used by lifetime PD)
- data/transition_matrices.py: _generate_model_consistent_matrix()
- models/credit_cycle.py: already fixed in previous commit

Added sign convention docs:
- vasicek.py conditional_pd() uses Basel convention (Z↑=loss↑)
- conditional_transition_matrix() uses Belkin convention (Z↑=호황)
- Both conventions documented in module docstrings

Pipeline 8/8 validation pass after fix

data/transition_matrices.py

@@ -128,18 +128,18 @@ def _generate_model_consistent_matrix(
             cum_prob_clipped = np.clip(cum_prob, 1e-10, 1.0 - 1e-10)
             thresholds[i, j] = norm.ppf(cum_prob_clipped)
     
-    # 2. Z-조건부 전이확률 계산
+    # 2. Z-조건부 전이확률 계산 (Belkin convention: Z>0 = 호황)
     cond_tm = np.zeros((n, n))
     for i in range(n - 1):
         for j in range(n):
             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)
             
             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)
             
             cond_tm[i, j] = max(upper - lower, 0.0)