feat(backtest): add DSL strategy skeleton and update __init__.py exports

- Add dsl_strategy decorator in dsl.py (P1 skeleton implementation)
- Update __init__.py to export BacktestEngine, Strategy, and related types
- All 106 backtest unit tests pass

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/easy_tdx/backtest/__init__.py

@@ -1 +1,36 @@
-"""easy_tdx.backtest — 向量化策略回测引擎（纯计算，零网络依赖）。"""
+"""easy_tdx.backtest — 向量化策略回测引擎（纯计算，零网络依赖）。
+
+快速开始::
+
+    from easy_tdx.backtest import BacktestEngine, Strategy
+
+    class MyStrategy(Strategy):
+        def init(self):
+            self.ma5 = self.I(MA, self.data.close, 5)
+            self.ma20 = self.I(MA, self.data.close, 20)
+
+        def next(self):
+            if crossover(self.ma5, self.ma20):
+                self.buy()
+            elif crossover(self.ma20, self.ma5):
+                self.sell()
+
+    engine = BacktestEngine(strategy=MyStrategy, cash=100000)
+    result = engine.run(df)
+    print(result.performance)
+"""
+
+from easy_tdx.backtest.engine import BacktestEngine  # noqa: F401
+from easy_tdx.backtest.strategy import Strategy, StrategyDataProxy, crossover  # noqa: F401
+from easy_tdx.backtest.types import BacktestResult, Position, Signal, Trade  # noqa: F401
+
+__all__ = [
+    "BacktestEngine",
+    "BacktestResult",
+    "Strategy",
+    "StrategyDataProxy",
+    "Signal",
+    "Trade",
+    "Position",
+    "crossover",
+]

src/easy_tdx/backtest/dsl.py

@@ -0,0 +1,53 @@
+"""DSL 策略定义模块 (P1 — 骨架)。
+
+v1 提供 @dsl_strategy 装饰器的基本实现。
+字符串 DSL 解析器将在后续版本实现。
+"""
+
+from __future__ import annotations
+
+from collections.abc import Callable
+from typing import Any
+
+import numpy as np
+
+from .strategy import Strategy
+
+
+def dsl_strategy(func: Callable[..., Any]) -> type[Strategy]:
+    """将函数编译为 Strategy 子类。
+
+    函数签名: (df: pd.DataFrame) -> tuple[np.ndarray[bool], np.ndarray[bool]]
+    返回 (buy_mask, sell_mask)。
+
+    用法::
+
+        @dsl_strategy
+        def dual_ma(df):
+            buy = CROSS(MA(df.close, 5), MA(df.close, 20))
+            sell = CROSS(MA(df.close, 20), MA(df.close, 5))
+            return buy, sell
+    """
+
+    class DSLStrategy(Strategy):
+        _signal_func = staticmethod(func)
+        _buy_mask: np.ndarray | None = None
+        _sell_mask: np.ndarray | None = None
+
+        def init(self) -> None:
+            pass
+
+        def next(self) -> None:
+            if self._buy_mask is None:
+                return
+            idx = self._bar_index
+            if idx < len(self._buy_mask) and self._buy_mask[idx]:
+                self.buy(size=0)
+            elif idx < len(self._sell_mask) and self._sell_mask[idx]:
+                self.sell(size=0)
+
+    DSLStrategy.__name__ = func.__name__
+    DSLStrategy.__qualname__ = func.__qualname__
+    DSLStrategy._signal_func = func  # type: ignore[attr-defined]
+
+    return DSLStrategy