feat: async TryEntry with goroutine, non-blocking main loop

- TryEntry now spawns a goroutine for order placement
- Main loop continues at 50-250ms even during entry
- 'entering' map prevents duplicate entries on same coin
- Async cleanup of entering state on completion

trader.go

@@ -118,6 +118,7 @@ type Trader struct {
 	db            *db.DB
 	mu            sync.Mutex
 	positions     map[string]*ArbPosition // coin -> position
+	entering      map[string]bool         // coin -> being entered (async goroutine)
 	lastTradeTime map[string]time.Time
 	closedTrades  []TradeRecord // history of closed trades (current session)
 
@@ -160,6 +161,7 @@ func NewTrader(cfg *Config, database *db.DB) *Trader {
 		bitget:         bt,
 		hyperliquid:    hl,
 		positions:      make(map[string]*ArbPosition),
+		entering:       make(map[string]bool),
 		lastTradeTime:  make(map[string]time.Time),
 	}
 
@@ -235,6 +237,8 @@ func (t *Trader) Tick(store *PriceStore, notifier *Notifier) {
 }
 
 // TryEntry opens initial position when threshold is met.
+// Returns true if entry was accepted (async goroutine will place orders).
+// Non-blocking — the main loop is not stalled by REST calls or the 300ms leg delay.
 func (t *Trader) TryEntry(opp *ArbOpportunity, store *PriceStore, notifier *Notifier) bool {
 	if !t.IsConfigured() {
 		return false
@@ -252,13 +256,25 @@ func (t *Trader) TryEntry(opp *ArbOpportunity, store *PriceStore, notifier *Noti
 		t.mu.Unlock()
 		return false
 	}
+	if t.entering[opp.Coin] {
+		t.mu.Unlock()
+		return false
+	}
 	if lastTime, ok := t.lastTradeTime[opp.Coin]; ok && time.Since(lastTime) < 30*time.Second {
 		t.mu.Unlock()
 		return false
 	}
+	t.entering[opp.Coin] = true
 	t.mu.Unlock()
 
-	return t.executeEntry(opp, store, notifier)
+	// Async goroutine — placeOrder calls (REST or mock) don't block the main loop
+	go func() {
+		t.executeEntry(opp, store, notifier)
+		t.mu.Lock()
+		delete(t.entering, opp.Coin)
+		t.mu.Unlock()
+	}()
+	return true
 }
 
 // executeEntry places both legs using the scan-time prices from ArbOpportunity.