fix: reversal = take profit, not stop loss

HL→BG position: long HL, short BG.
When bgP and hlP cross over (spread reverses):
- Long HL: price went up → profit
- Short BG: price went down → profit
Both legs profit simultaneously. Reversal is MAX profit moment.

trader.go

@@ -457,7 +457,7 @@ func (t *Trader) checkExit(pos *ArbPosition, bgP, hlP, diffPct float64, notifier
 	// Stop-loss: spread reversed (went negative)
 	if diffPct < -0.02 {
 		shouldExit = true
-		exitReason = "价差反转,止损平仓"
+		exitReason = "价差反转,止盈平仓"
 	}
 
 	if elapsed > 30*time.Minute {