feat(rotation): componentize position weighting + fix bond threshold consistency

- Extract compute_position_weights() as pluggable pure function
- Add WeightType enum (equal/rank) and RotationConfig.weight field
- Fix bond threshold dimension mismatch: use configured factor function
  for all assets instead of hardcoded weighted_momentum_score
- Default weight: equal in config, active: rank in config_simple.yaml

rotation/config_loader.py

@@ -49,6 +49,12 @@ class ThresholdMode(str, Enum):
     DYNAMIC = "dynamic"     # 动态阈值
 
 
+class WeightType(str, Enum):
+    """仓位加权模式"""
+    EQUAL = "equal"         # 等权
+    RANK = "rank"           # 按排名加权 (slot i gets (N-i)/triangular(N))
+
+
 class DataSourceType(str, Enum):
     """数据源类型"""
     FLASK_API = "flask_api"
@@ -169,6 +175,7 @@ class RotationConfig(BaseModel):
     select_num: int = Field(default=3, ge=1, le=10)
     diversified: bool = Field(default=True)
     threshold: ThresholdConfig = Field(default_factory=ThresholdConfig)
+    weight: WeightType = Field(default=WeightType.EQUAL)
 
 
 class RebalanceConfig(BaseModel):