feat(macro): comprehensive variable exploration, R²=0.028→0.747
- New: data/macro_analysis.py (15 base × 6 transforms = 116 candidates)
- Top correlations: CORP_AA_LOGR(r=-0.75), credit spread, term spread
- Exhaustive 3-var search (1749 combos), best adj.R²=0.71
- Modified: data/macro_data.py
- Added GOVT_3Y, CORP_AA, CORP_BBB ECOS queries + fallback data
- New: compute_derived_features() for optimal 3 predictors
- Modified: main.py
- Computes derived features + passes combined input to stepwise
- Scenario paths now include derived features for prediction
- Selected 3 variables: CORP_AA_LOGR, CPI_GROWTH, CREDIT_SPREAD_LAG1
- All 8/8 validation tests pass (incl. R² now Pass)
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