Fix: Correct Sharpe Ratio calculation by using proper equity values

Critical bug fix in Sharpe Ratio calculation logic:

Problem:
- Previously calculated equity as TotalBalance + TotalUnrealizedProfit
- This was incorrect because TotalBalance already stores TotalEquity
- TotalUnrealizedProfit actually stores TotalPnL (not unrealized profit)
- This caused: equity = 2 * TotalEquity - InitialBalance (wrong!)

Root cause:
- Field naming mismatch between AccountSnapshot and actual stored values
- TotalBalance field actually contains TotalEquity (wallet + unrealized)
- TotalUnrealizedProfit field actually contains TotalPnL (equity - initial)

Solution:
- Use TotalBalance directly as it already represents complete account equity
- Added clear comments explaining the field name vs content mismatch
- Sharpe Ratio now correctly calculates risk-adjusted returns

Impact:
- Sharpe Ratio values are now mathematically accurate
- AI performance assessment is now reliable
- No changes needed to data storage or API layer

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

logger/decision_logger.go

@@ -494,9 +494,12 @@ func (l *DecisionLogger) calculateSharpeRatio(records []*DecisionRecord) float64
 	}
 
 	// 提取每个周期的账户净值
+	// 注意：TotalBalance字段实际存储的是TotalEquity（账户总净值）
+	// TotalUnrealizedProfit字段实际存储的是TotalPnL（相对初始余额的盈亏）
 	var equities []float64
 	for _, record := range records {
-		equity := record.AccountState.TotalBalance + record.AccountState.TotalUnrealizedProfit
+		// 直接使用TotalBalance，因为它已经是完整的账户净值
+		equity := record.AccountState.TotalBalance
 		if equity > 0 {
 			equities = append(equities, equity)
 		}