feat: 新增slope_r2因子并切换为默认因子（年化19.84%, 夏普1.14）

- simple_rotation.py: 新增3种score函数（vol_adjusted_momentum, slope_r2, momentum）
- config_loader.py: FactorType枚举新增VOL_ADJUSTED_MOMENTUM
- config_simple.yaml: factor.type 切换为 slope_r2
- experiments/factor_comparison.py: 4种因子对比实验脚本
- experiments/output: 实验结果（slope_r2全面胜出）

rotation/config_loader.py

@@ -33,6 +33,7 @@ class FactorType(str, Enum):
     MOMENTUM = "momentum"
     SLOPE_R2 = "slope_r2"
     WEIGHTED_MOMENTUM = "weighted_momentum"
+    VOL_ADJUSTED_MOMENTUM = "vol_adjusted_momentum"
 
 
 class PremiumMode(str, Enum):