fix: async entry race condition — 'entering' status prevents premature exit

During async executeEntry goroutine, the 300ms sleep between legs
left the position vulnerable to checkExit() from the main loop.
Added intermediate Status='entering' that checkExit/checkScaleIn
skip ('entering' != 'open'), switched to 'open' only after both
legs are placed.

trader.go

@@ -47,7 +47,7 @@ type ArbPosition struct {
 	LastScaleAt time.Time // when we last scaled in
 	StartedAt   time.Time
 	ExitedAt    time.Time
-	Status      string // "open", "closed"
+	Status      string // "entering", "open", "closed"
 	RealizedPnl float64
 	ErrorLog    string
 
@@ -311,7 +311,7 @@ func (t *Trader) executeEntry(opp *ArbOpportunity, store *PriceStore, notifier *
 		Coin:        opp.Coin,
 		AmountUSD:   t.cfg.TradeAmountUSD,
 		StartedAt:   time.Now(),
-		Status:      "open",
+		Status:      "entering", // prevent checkExit/checkScaleIn during leg placement
 		ScaleLevels: 0,
 	}
 
@@ -361,6 +361,7 @@ func (t *Trader) executeEntry(opp *ArbOpportunity, store *PriceStore, notifier *
 	}
 
 	pos.LastScaleAt = time.Now()
+	pos.Status = "open" // both legs placed, ready for Tick/exit logic
 
 	log.Printf("[Trader] %s: Opened %s | Long %s @ %.2f  Short %s @ %.2f | $%.0f",
 		pos.Coin, pos.Direction, pos.LongLeg.Exchange, pos.LongLeg.EntryPrice,