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_simple.yaml

@@ -108,6 +108,7 @@ rebalance:
 rotation:
   diversified: true
   select_num: 3
+  weight: rank
   threshold:
     dynamic:
       fallback_enabled: true