feat: actual exchange fees for entry orders

- BitgetTrade: added GetTradeFee() queries /mix/order/fills
- HyperLiquidTrade: added GetTradeFee() parses MarketOpen response
- trader.placeOrder() now returns (errMsg, actualFeeUSD)
- executeEntry uses actual fee from exchange, falls back to estimate

exchange/bitget_trade.go

@@ -87,6 +87,57 @@ func (b *BitgetTrade) PlaceMarketOrder(side, symbol, size, tradeSide string) (st
 	return result.Data.OrderID, nil
 }
 
+// GetTradeFee queries the fills endpoint for actual fee charged.
+func (b *BitgetTrade) GetTradeFee(symbol, orderID string) (feeUSD float64, err error) {
+	ts := fmt.Sprintf("%d", time.Now().UnixMilli())
+	method := "GET"
+	requestPath := "/api/v2/mix/order/fills?symbol=" + symbol + "&orderId=" + orderID
+	host := "https://api.bitget.com"
+
+	sign := b.sign(method, requestPath, ts, "")
+	url := host + requestPath
+	req, _ := http.NewRequest(method, url, nil)
+	req.Header.Set("ACCESS-KEY", b.APIKey)
+	req.Header.Set("ACCESS-SIGN", sign)
+	req.Header.Set("ACCESS-TIMESTAMP", ts)
+	req.Header.Set("ACCESS-PASSPHRASE", b.Passphrase)
+	if b.paperMode {
+		req.Header.Set("paptrading", "1")
+	}
+
+	resp, err := b.client.Do(req)
+	if err != nil {
+		return 0, fmt.Errorf("http: %w", err)
+	}
+	defer resp.Body.Close()
+	respBody, _ := io.ReadAll(resp.Body)
+
+	var raw struct {
+		Code string            `json:"code"`
+		Msg  string            `json:"msg"`
+		Data []json.RawMessage `json:"data"`
+	}
+	if err := json.Unmarshal(respBody, &raw); err != nil {
+		return 0, fmt.Errorf("parse: %s", string(respBody))
+	}
+	if raw.Code != "00000" {
+		return 0, fmt.Errorf("bitget error: %s - %s", raw.Code, raw.Msg)
+	}
+
+	var totalFee float64
+	for _, item := range raw.Data {
+		var fill struct {
+			FillFee string `json:"fillFee"`
+		}
+		if err := json.Unmarshal(item, &fill); err != nil {
+			continue
+		}
+		f, _ := strconv.ParseFloat(fill.FillFee, 64)
+		totalFee += math.Abs(f)
+	}
+	return totalFee, nil
+}
+
 func (b *BitgetTrade) sign(method, requestPath, timestamp, body string) string {
 	raw := timestamp + method + requestPath + body
 	mac := hmac.New(sha256.New, []byte(b.APISecret))
@@ -128,7 +179,6 @@ func (b *BitgetTrade) GetBalance() (float64, error) {
 		return 0, fmt.Errorf("bitget error: %s - %s", code, msg)
 	}
 
-	// Parse data as array of accounts
 	dataRaw, ok := raw["data"]
 	if !ok || dataRaw == nil {
 		return 0, fmt.Errorf("no data in response")