fix: data race, scale-in PnL, nonce mutex, dead code, hourly check

- 🔴 Data race: Add GetPositionsCopy() returning deep copies (no shared
  ArbPosition pointers). Use it in dashboard broadcastLoop + handleStatus.
- 🟡 Scale-in PnL: Track LongEntryPrices/ShortEntryPrices on ArbPosition,
  compute weighted average (harmonic mean) at exit for accurate PnL.
- 🟢 CalcNetProfit: Delete dead code from exchange/helpers.go.
- 🟢 HL nonce: Add sync.Mutex around lastNonce++ (thread safety).
- 🟢 Hourly check: Change from 5-second window to minute window.
- 🟢 ExitPrice: Test mode closeLeg already handled by checkExit.

exchange/hyperliquid_trade.go

@@ -8,6 +8,7 @@ import (
 	"math/big"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 
 	"crypto/ed25519"
@@ -50,6 +51,7 @@ type HyperLiquidTrade struct {
 	Address    string
 	client     *http.Client
 	lastNonce  int64
+	nonceMu    sync.Mutex // protect lastNonce++ (Issue #4)
 }
 
 func NewHyperLiquidTrade(privateKeyHex, address string) (*HyperLiquidTrade, error) {
@@ -96,9 +98,11 @@ func (h *HyperLiquidTrade) PlaceMarketOrder(coin, side, sz string) (string, erro
 		BrokerCode: 0,
 	}
 
-	// Generate nonce
+	// Generate nonce (thread-safe)
+	h.nonceMu.Lock()
 	h.lastNonce++
 	nonce := time.Now().UnixMilli()*1_000_000 + h.lastNonce%1_000_000
+	h.nonceMu.Unlock()
 
 	// Sign the action
 	sig, err := h.signAction(action, nonce)