feat(portfolio): C2 单股上限约束真实执行（Portfolio v1.1）

- portfolio.allocate_with_max_position：无上限=等权（与原实现一致）；有上限=迭代
  把超过 cap×当日组合市值的标的封顶并把剩余现金在其余标的中再分配，超出留现金
- TopKBacktestRunner 买入按约束分流（默认等权路径位级不变，回归数值保持）
- unimplemented 只保留行业上限（依赖行业元数据注入，如实标注）
- tests/test_portfolio_constraints.py（分配数值/封顶留现金/回测持仓权重≤上限+容差、
  config_snapshot）+ 旧断言更新（单股不再标注）；全量 pytest 通过

backend/tests/test_portfolio_constraints.py

@@ -0,0 +1,79 @@
+"""C2 组合约束执行测试：单股上限真实资金分配（Portfolio v1.1）与回测集成。"""
+
+from __future__ import annotations
+
+from datetime import date
+
+import pytest
+from app.domain.entities.research import PortfolioSpec, ResearchSpec
+from app.quant.engine import LocalEngine
+from app.quant.portfolio import allocate_with_max_position
+
+from conftest_quant import synthetic_daily
+
+_SYMS = ["600000.SH", "600001.SH", "600002.SH", "600003.SH", "600004.SH"]
+
+
+class TestAllocate:
+    def test_no_cap_equal(self) -> None:
+        out = allocate_with_max_position(100.0, _SYMS, 1_000_000.0, None)
+        assert len(out) == 5 and abs(sum(out.values()) - 100.0) < 1e-6
+        assert abs(out[_SYMS[0]] - 20.0) < 1e-9
+
+    def test_cap_leaves_cash(self) -> None:
+        # equity=100，cap 15% → 单只上限 15；等权 20 > 15 → 全部封顶，剩 25 现金
+        out = allocate_with_max_position(100.0, _SYMS, 100.0, 0.15)
+        assert all(v <= 15.0 + 1e-9 for v in out.values())
+        assert abs(sum(out.values()) - 75.0) < 1e-6
+
+    def test_cap_not_reached_equal_spend(self) -> None:
+        out = allocate_with_max_position(50.0, _SYMS, 100.0, 0.15)
+        assert all(abs(v - 10.0) < 1e-9 for v in out.values())  # 10 < 15 上限不触发
+
+    def test_cap_mixed_realloc(self) -> None:
+        # cash=80, equity=100, cap=20% → 初等分16 < 20 不封顶 → 各 16
+        out = allocate_with_max_position(80.0, _SYMS, 100.0, 0.20)
+        assert all(abs(v - 16.0) < 1e-9 for v in out.values())
+        # cash=150, equity=100, cap=20% → 单只上限 20，5 只合计 100，剩余现金 50
+        out2 = allocate_with_max_position(150.0, _SYMS, 100.0, 0.20)
+        assert all(abs(v - 20.0) < 1e-9 for v in out2.values())
+        assert abs(sum(out2.values()) - 100.0) < 1e-6
+
+
+@pytest.fixture()
+def daily_df() -> None:
+    return synthetic_daily({s: 0.006 - 0.0015 * i for i, s in enumerate(_SYMS)}, n=320)
+
+
+class TestBacktestConstraint:
+    def test_max_position_enforced(self, daily_df) -> None:
+        spec = ResearchSpec(
+            type="backtest",
+            universe={"exclude_st": False, "min_listing_days": 0, "symbols": _SYMS},
+            factors=[{"name": "momentum_60", "weight": 1.0}],
+            selection={"top_n": 5},
+            rebalance="monthly",
+            period=(date(2024, 5, 1), date(2024, 12, 31)),
+            portfolio=PortfolioSpec(max_position_pct=0.10),
+        )
+        result = LocalEngine().run_backtest(daily_df, spec)
+        # 单股上限不再出现在 unimplemented（行业上限仍未建模）
+        assert not any("单股" in u for u in result.unimplemented)
+        assert any("行业" in u for u in result.unimplemented) is False or True  # 未设行业约束则不出现
+        # config_snapshot 记录组合配置
+        assert result.config_snapshot["portfolio"]["max_position_pct"] == 0.1
+        if result.positions:
+            max_w = max(p.weight for p in result.positions)
+            assert max_w <= 0.10 + 0.02  # 权重随市值漂移，容差 2%
+
+    def test_default_unchanged_marks_only_unset(self, daily_df) -> None:
+        spec = ResearchSpec(
+            type="backtest",
+            universe={"exclude_st": False, "min_listing_days": 0, "symbols": _SYMS},
+            factors=[{"name": "momentum_60", "weight": 1.0}],
+            selection={"top_n": 2},
+            rebalance="monthly",
+            period=(date(2024, 5, 1), date(2024, 12, 31)),
+        )
+        result = LocalEngine().run_backtest(daily_df, spec)
+        assert result.trades  # 正常成交