feat(engine): 添加核心基础设施 — engine/common 模块

- engine/__init__.py: 包入口，导出 Kline/KlineInterval/OrderBook/Ticker/Trade
- common/base.py: BaseStrategy 抽象基类，定义 on_kline/on_ticker/on_orderbook 回调
- common/models.py: Pydantic 数据模型，与 TS 侧 types 字段对齐，支持字段校验
- common/config.py: 全局配置加载（YAML），统一 engine/env.yaml 读取
- common/logger.py: 结构化日志，支持 JSON/pretty print 输出

engine/__init__.py

@@ -0,0 +1,11 @@
+# engine - 策略引擎模块
+
+from .common import Kline, KlineInterval, OrderBook, Ticker, Trade, config
+from .data import DataService
+from .backtest import BacktestEngine, BacktestConfig, BacktestResult, BacktestMetrics, BacktestTrade
+
+__all__ = [
+    "Kline", "KlineInterval", "OrderBook", "Ticker", "Trade",
+    "DataService", "config",
+    "BacktestEngine", "BacktestConfig", "BacktestResult", "BacktestMetrics", "BacktestTrade",
+]