feat(backtest): 接真实 CTA 策略跑通端到端回测（DoubleMaStrategy on 600000）

修复 cta_engine 在真数据上的多个 bug（Phase 2 未在真数据验证）：
- interval "1d" -> "d"（vnpy Interval.DAILY.value）
- capital 0 -> 1_000_000（0 致首笔交易即爆仓，统计全 0）
- statistics 改用 calculate_statistics(df)（旧代码误用 calculate_result 拿 DataFrame）
- statistics JSON-safe（vnpy 可能含 Timestamp）
- test_cta_engine mock 匹配新流程（calculate_statistics 返回统计字典）

验证：diag_cta.py 真实回测 DoubleMaStrategy on 600000 (2024H1, 111 天)
→ 真实统计 total_return -0.017% / sharpe -1.03 / max_drawdown -2.17 / 1 trade
容器 79 tests passed。

scripts/diag_cta.py

@@ -0,0 +1,51 @@
+"""Diagnostic: real CTA backtest of DoubleMaStrategy on 600000 (A-share daily).
+Guarded entry for spawn. Throwaway."""
+import sys
+import os
+import traceback
+
+_VNPY_SRC = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", "vnpy_v4.4.0"))
+_REPO = os.path.dirname(_VNPY_SRC)
+for _p in (_REPO, _VNPY_SRC):
+    if _p not in sys.path:
+        sys.path.insert(0, _p)
+
+
+def main():
+    # Configure vnpy DB to the real quant_trading.db BEFORE engine.load_data()
+    from vnpy.trader.setting import SETTINGS
+    from sanguo_data.config import load_config
+    cfg = load_config("/app/config/data_platform.yaml")
+    SETTINGS["database.name"] = "sqlite"
+    SETTINGS["database.database"] = cfg.data_paths["vnpy_db"]
+    print("DB:", SETTINGS["database.database"])
+
+    from vnpy_ctastrategy.strategies.double_ma_strategy import DoubleMaStrategy
+    print("DoubleMaStrategy.parameters:", getattr(DoubleMaStrategy, "parameters", "?"))
+
+    from sanguo_backtest.cta_engine import run_cta_backtest
+    # Classic double-MA params (vnpy example defaults); fixed_size=1
+    params = {"fast_window": 10, "slow_window": 20, "fixed_size": 1}
+
+    print(f"run_cta_backtest DoubleMaStrategy on 600000, 2024-01-01..2024-06-30, params={params}")
+    try:
+        result = run_cta_backtest(
+            DoubleMaStrategy, "600000", params,
+            "2024-01-01", "2024-06-30", cfg, "/tmp/cta_results.db",
+        )
+        print("=== status:", result.status)
+        print("=== statistics ===")
+        if result.statistics:
+            for k, v in result.statistics.items():
+                print(f"  {k}: {v}")
+        else:
+            print("  (empty)")
+        if result.status == "failed":
+            print("=== error ===")
+            print(result.error_msg)
+    except Exception:
+        traceback.print_exc()
+
+
+if __name__ == "__main__":
+    main()