fix(grid): use actual PositionSize sum instead of count in syncGridState heuristic

The position-based heuristic was using `float64(previousFilledCount) * level.OrderQuantity`
which incorrectly assumed uniform order quantities. Since the grid uses weighted distribution
(gaussian, pyramid, uniform) where orders have different quantities, this could lead to
incorrect fill detection.

Now sums the actual PositionSize from filled levels for accurate comparison.
Also adds warning log when GetPositions() fails.

trader/auto_trader_grid.go

@@ -814,7 +814,9 @@ func (at *AutoTrader) syncGridState() {
 	// Get current positions to verify fills
 	positions, err := at.trader.GetPositions()
 	currentPositionSize := 0.0
-	if err == nil {
+	if err != nil {
+		logger.Warnf("[Grid] Failed to get positions for state sync: %v", err)
+	} else {
 		for _, pos := range positions {
 			if sym, ok := pos["symbol"].(string); ok && sym == gridConfig.Symbol {
 				if size, ok := pos["positionAmt"].(float64); ok {
@@ -826,10 +828,10 @@ func (at *AutoTrader) syncGridState() {
 
 	// Update levels based on order status
 	at.gridState.mu.Lock()
-	previousFilledCount := 0
+	expectedPositionSize := 0.0
 	for _, level := range at.gridState.Levels {
 		if level.State == "filled" {
-			previousFilledCount++
+			expectedPositionSize += level.PositionSize
 		}
 	}
 
@@ -839,7 +841,8 @@ func (at *AutoTrader) syncGridState() {
 			if !activeOrderIDs[level.OrderID] {
 				// Order no longer exists - check if position changed to determine fill vs cancel
 				// This is a heuristic - ideally we'd query order history
-				if math.Abs(currentPositionSize) > math.Abs(float64(previousFilledCount)*level.OrderQuantity) {
+				// If current position is larger than expected filled positions, this order was likely filled
+				if math.Abs(currentPositionSize) > math.Abs(expectedPositionSize) {
 					// Position increased, likely filled
 					level.State = "filled"
 					level.PositionEntry = level.Price