feat: 新增 standardized_slope (t-statistic) 因子并实验验证

- simple_rotation.py: 新增 standardized_slope_score 函数 (slope/SE)
- config_loader.py: FactorType 枚举新增 STANDARDIZED_SLOPE
- 对比实验结果: standardized_slope 年化 13.73% vs slope_r2 19.84%
- 结论: t-statistic 过度惩罚高波动资产的有效趋势信号，不适合本场景
- 文档更新: 动量因子对比调研报告新增 3.3 节详细分析

rotation/experiments/output/std_slope_test_results.json

@@ -0,0 +1,25 @@
+{
+  "timestamp": "2026-06-06T16:36:39.736366",
+  "results": [
+    {
+      "factor_type": "slope_r2",
+      "annual_return": 0.198416094188119,
+      "total_return": 2.0421974188211456,
+      "sharpe_ratio": 1.1350010914615083,
+      "max_drawdown": -0.15352659557851117,
+      "win_rate": 0.541343669250646,
+      "rebalance_count": 394,
+      "calmar_ratio": 1.2923890707043786
+    },
+    {
+      "factor_type": "standardized_slope",
+      "annual_return": 0.13732579856023497,
+      "total_return": 1.2055386092808908,
+      "sharpe_ratio": 1.0139515617271433,
+      "max_drawdown": -0.13523854511100616,
+      "win_rate": 0.5452196382428941,
+      "rebalance_count": 335,
+      "calmar_ratio": 1.0154338650087928
+    }
+  ]
+}