feat: Lifetime PD (50yr) - Belkin & Suchower + Vasicek model

- Belkin & Suchower (1998) credit cycle index (Zt) estimation via WLS
- Vasicek single-factor conditional PD/TM model
- Macro-Zt OLS regression with stepwise variable selection
- 3-scenario (boom/neutral/recession) 50yr PD projection
- Statistical validation suite (ADF, Ljung-Box, R2, ARCH)
- BOK ECOS API integration with fallback data
- Visualization module (7 chart types)
- Detailed theoretical methodology docs/methodology.md

.gitignore

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+# Python
+__pycache__/
+*.py[cod]
+*$py.class
+*.egg-info/
+dist/
+build/
+
+# Results (regenerable)
+results/*.png
+results/*.csv
+
+# IDE
+.vscode/
+.idea/
+*.swp
+
+# OS
+Thumbs.db
+.DS_Store
+
+# Env
+.env
+*.env
+
+# Data
+data/raw/*.csv
+data/raw/*.xlsx