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/app/quant/local_engine.py

@@ -32,6 +32,7 @@ from app.quant.composite import (  # noqa: F401 —— re-export（模块化后
     cross_sectional_zscore,
 )
 from app.quant.evaluation import run_factor_test
+from app.quant.portfolio import equal_weight_budget, unimplemented_notes
 
 TRADING_DAYS = 252
 _DEFAULT_UNIMPLEMENTED = [
@@ -169,7 +170,7 @@ class TopKBacktestRunner:
             targets.append(s)
 
         if targets:
-            budget = cash / len(targets)
+            budget = equal_weight_budget(cash, len(targets))
             for s in targets:
                 c = float(close_d[s])
                 price_in = c * (1 + self.costs.slippage_rate)
@@ -263,7 +264,7 @@ class TopKBacktestRunner:
             positions=positions,
             trades=trades,
             turnover_pct=round(sum(notional) / max(init, 1) * 100, 2),
-            unimplemented=list(_DEFAULT_UNIMPLEMENTED),
+            unimplemented=list(_DEFAULT_UNIMPLEMENTED) + unimplemented_notes(self.spec.portfolio),
             config_snapshot=self.spec.model_dump(mode="json"),
         )