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

requirements.txt

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+numpy>=1.24
+scipy>=1.10
+pandas>=2.0
+statsmodels>=0.14
+matplotlib>=3.7
+seaborn>=0.12
+requests>=2.28
+pyyaml>=6.0
+openpyxl>=3.1
+tabulate>=0.9
+pytest>=7.0