feat(trader): C期分期项收尾—资金占用成本+分红送股+_restore_ledger修复

- 资金占用成本(spec§195): StrategyRunner.daily_borrow_cost(used×risk_free/365)
  归因per_strategy_pnl(不碰account总账, account.equity真实净值不变);
  config risk_free_rate=0.02; engine.step mark_to_market后计扣; =0向后兼容跳过
- 分红送股(spec§295): dividend_source.py(akshare stock_history_dividend_detail,
  实测600000/000001纯现金分红); PositionLedger.apply_split(volume×factor/avg÷factor);
  Account.apply_cash_dividend; engine._apply_dividends(除权日调整,现金先split后);
  mark_to_market停牌prev_close兜底(今收→前收→均价); _run_replay注入dividends日历
- 修_restore_ledger预存bug: PositionLedger.__init__加volume/frozen/avg_price参数
  (原只symbol, live_orchestrator跨日恢复4参数调用会TypeError, 首次step空仓未暴露)
- 139 passed(119基准+20分红+3占用成本), 无回归
- live_step dividends注入待分期项(每日拉全市场分红慢, 需run_daily_update预拉日历)

config/data_platform.yaml

@@ -36,3 +36,6 @@ performance:
   max_retries: 3
   fail_window: 100
   fail_threshold: 0.8
+
+# 资金占用成本归因（spec §195）：年化无风险利率，每策略占用资金按此日扣归因到 PnL
+risk_free_rate: 0.02