refactor(quant): M7.2a Composite Engine 模块化（quant/composite.py）

- cross_sectional_zscore / composite_score / build_factor_panels 从 local_engine 迁入
  quant/composite.py；新增统一入口 build_score_panel(daily, factor_specs)
- local_engine re-export 保持旧引用兼容；selection/engine 的评分面板构建均指向
  composite —— 选股与回测的复合分实现收敛于一处
- 回归：quant/eval/research/selection 一致性/qlib 引擎测试全过；全量 pytest 通过

backend/tests/test_research_eval.py

@@ -14,9 +14,10 @@ from app.domain.entities.research import (
     SelectionSpec,
     UniverseSpec,
 )
+from app.quant.composite import composite_score, cross_sectional_zscore
 from app.quant.evaluation import run_factor_test
 from app.quant.factors import compute_factor
-from app.quant.local_engine import composite_score, cross_sectional_zscore, rebalance_dates
+from app.quant.local_engine import rebalance_dates
 from pydantic import ValidationError
 
 from conftest_quant import synthetic_daily
@@ -115,7 +116,7 @@ class TestEvaluation:
 
 class TestSingleStockDegradation:
     def test_zscore_single_stock_keeps_candidate(self) -> None:
-        from app.quant.local_engine import cross_sectional_zscore
+        from app.quant.composite import cross_sectional_zscore
 
         daily = synthetic_daily({"ONLY": 0.001}, n=80)
         _d, panel = compute_factor("momentum_20", daily)