fix(trader): stop over-attributing entry fees on partial position closes

The FIFO matcher reduced an open trade's remaining quantity but not its
remaining fee, so each subsequent partial close re-attributed entry fee
that earlier closes had already counted (e.g. open 2.0 with fee 0.4,
two 1.0 closes attributed 0.6 total). Deduct the consumed fee portion
alongside the quantity so attributed fees sum to the fee actually paid.

trader/position_rebuild.go

@@ -143,7 +143,11 @@ func buildClosedPosition(trade TradeRecord, side string, state *positionState) *
 
 			weightedSum += ot.Price * matchQty
 			matchedQty += matchQty
-			totalEntryFee += ot.Fee * (matchQty / ot.Quantity)
+			// Attribute the entry fee proportionally and deduct the consumed
+			// portion from the open trade, so a later partial close cannot
+			// re-attribute fee that was already counted.
+			feePortion := ot.Fee * (matchQty / ot.Quantity)
+			totalEntryFee += feePortion
 
 			if entryTime.IsZero() {
 				entryTime = ot.Time
@@ -151,6 +155,7 @@ func buildClosedPosition(trade TradeRecord, side string, state *positionState) *
 
 			remainingQty -= matchQty
 			ot.Quantity -= matchQty
+			ot.Fee -= feePortion
 
 			// Remove fully consumed open trade
 			if ot.Quantity <= dustQuantityEpsilon {