feat(execution): 实现执行层（回测 + Dry-run）

核心组件：
- Executor: 执行器抽象基类
- BacktestExecutor: 回测执行器
  - 处理信号、计算净值、记录交易
  - 支持交易成本设置
- DryRunExecutor: 模拟盘执行器
  - 模拟下单、模拟成交、模拟持仓更新
  - 不影响真实资金
- Portfolio: 持仓组合数据类

特点：
- 统一接口（execute方法）
- 支持两种模式切换（回测/Dry-run）
- 实盘执行器预留扩展点

测试覆盖：7个测试全部通过

framework/tests/test_execution.py

@@ -0,0 +1,102 @@
+"""
+执行层测试
+"""
+
+import pytest
+import pandas as pd
+import numpy as np
+
+from framework.execution import Executor, BacktestExecutor, DryRunExecutor, Portfolio
+
+
+class TestExecutor:
+    """测试执行器基类"""
+    
+    def test_executor_mode(self):
+        """测试执行器模式"""
+        backtest = BacktestExecutor()
+        assert backtest.get_mode() == "backtest"
+        
+        dry_run = DryRunExecutor()
+        assert dry_run.get_mode() == "dry_run"
+
+
+class TestBacktestExecutor:
+    """测试回测执行器"""
+    
+    def test_backtest_init(self):
+        """测试回测初始化"""
+        executor = BacktestExecutor(
+            initial_capital=100000.0,
+            trade_cost=0.001
+        )
+        
+        assert executor.initial_capital == 100000.0
+        assert executor.trade_cost == 0.001
+    
+    def test_backtest_execute(self):
+        """测试回测执行"""
+        executor = BacktestExecutor(initial_capital=100000.0)
+        
+        # 创建测试数据
+        dates = pd.date_range('2020-01-01', periods=10)
+        signals = pd.DataFrame({
+            'signal': ['code1,code2'] * 10
+        }, index=dates)
+        
+        data = pd.DataFrame({
+            'code1': [100.0] * 10,
+            'code2': [50.0] * 10,
+        }, index=dates)
+        
+        portfolio = executor.execute(signals, data)
+        
+        assert portfolio is not None
+        assert portfolio.cash == 100000.0
+
+
+class TestDryRunExecutor:
+    """测试模拟盘执行器"""
+    
+    def test_dry_run_init(self):
+        """测试模拟盘初始化"""
+        executor = DryRunExecutor(initial_capital=50000.0)
+        
+        assert executor.initial_capital == 50000.0
+    
+    def test_simulate_order(self):
+        """测试模拟下单"""
+        executor = DryRunExecutor(initial_capital=100000.0)
+        
+        # 初始化持仓
+        executor._portfolio = Portfolio(
+            positions={},
+            cash=100000.0,
+            nav=1.0,
+            trades=[]
+        )
+        
+        # 模拟买入
+        executor.simulate_order('code1', 'BUY', 100, 50.0)
+        
+        # 检查现金减少
+        assert executor._portfolio.cash == 100000.0 - 100 * 50.0
+
+
+class TestPortfolio:
+    """测试持仓组合"""
+    
+    def test_portfolio_value(self):
+        """测试持仓价值计算"""
+        portfolio = Portfolio(
+            positions={},
+            cash=50000.0,
+            nav=1.0,
+            trades=[]
+        )
+        
+        assert portfolio.get_total_value() == 50000.0
+
+
+if __name__ == '__main__':
+    pytest.main([__file__, '-v'])