feat(portfolio): M8.2 Portfolio Engine 模块化（等权收敛 + 约束显式标注）

- research.PortfolioSpec（weighting=equal；max_position_pct/max_industry_weight_pct 预留）
  + ResearchSpec.portfolio；config_snapshot 自动记录组合配置
- quant/portfolio.py：equal_weight_budget（与既有等权回测语义一致，行为收敛到本模块）+
  unimplemented_notes（设置约束即在结果中显式标注未建模，禁止假装支持）
- TopKBacktestRunner 预算与 unimplemented 改用 portfolio 模块；默认配置数值不变
  （一致性/quant 引擎回归通过）；tests 补约束标注与 config_snapshot；全量 pytest 通过

backend/tests/test_selection_backtest_consistency.py

@@ -123,3 +123,26 @@ class TestSelectionBacktestConsistency:
             held, key=lambda s: res.candidates[[x.symbol for x in res.candidates].index(s)].score,
             reverse=True,
         )
+
+
+class TestPortfolioEngine:
+    def test_equal_weight_default_unchanged(self, daily_df) -> None:
+        """新增 PortfolioSpec 后默认配置回测结果与未设置前一致（回归由本文件首测已锁数值）。"""
+        from app.domain.entities.research import PortfolioSpec
+        from app.quant.engine import LocalEngine
+
+        spec = _spec(portfolio=PortfolioSpec())
+        result = LocalEngine().run_backtest(daily_df, spec)
+        assert result.summary.total_trades >= 0
+        # 未设约束 → 无组合约束说明
+        assert not any("约束未建模" in u for u in result.unimplemented)
+
+    def test_constraint_declared_in_unimplemented(self, daily_df) -> None:
+        from app.domain.entities.research import PortfolioSpec
+        from app.quant.engine import LocalEngine
+
+        spec = _spec(portfolio=PortfolioSpec(max_position_pct=0.1))
+        result = LocalEngine().run_backtest(daily_df, spec)
+        assert any("最大单股权重" in u for u in result.unimplemented)
+        # config_snapshot 记录组合配置
+        assert result.config_snapshot["portfolio"]["max_position_pct"] == 0.1