feat(model): KAP YTM PD floor integration, expanded 226-var search, ADF fix (AIC->BIC), Model#2 with 6-test diagnostics

- Replace hardcoded DEFAULT_PD_FLOORS with build_complete_pd_floor_table() (KAP bond YTM)
- Fix ADF test: autolag='AIC' -> 'BIC' for small sample (N=26) robustness
- Expand variable search: 40 -> 226 vars (log/diff/return/lag2), 1.9M combos
- Select Model #2: HOUSING_PRICE + CREDIT_SPREAD_LAG1 + CURRENT_ACCOUNT_R
- Add 6-test diagnostics table to AR1 sheet (ADF/LB/DW/BP/ARCH/Shapiro)
- Add Korean variable names for transformed variables
- Generate report v7 with full diagnostics

validation/statistical_tests.py

@@ -44,7 +44,11 @@ def test_stationarity(
     -------
     dict with test_statistic, p_value, critical_values, is_stationary
     """
-    result = adfuller(series, autolag="AIC")
+    # BIC를 사용하는 이유:
+    # - AIC는 소표본(N<50)에서 과다 lag 선택 경향 (Hamilton 1994, Ch.17)
+    # - N=26에서 AIC → lag=8 → 유효관측치=17 → 검정력 상실
+    # - BIC는 보수적 lag 선택 → 소표본에서 적절 (Schwarz 1978)
+    result = adfuller(series, autolag="BIC")
     
     is_stationary = result[1] < significance