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 {

trader/position_rebuild_test.go

@@ -154,12 +154,15 @@ func TestRebuildPositionsFromTrades_PartialClose(t *testing.T) {
 	if !floatsClose(records[0].Fee, 0.3) {
 		t.Errorf("records[0].Fee = %v, want 0.3", records[0].Fee)
 	}
-	// NOTE: documents current behavior. The open trade's Fee field is not
-	// reduced when partially consumed, so the second close re-attributes the
-	// full remaining ratio of the original fee: 0.1 + 0.4*(1/1) = 0.5.
-	// Total attributed entry fee across both closes is 0.6 > 0.4 actually paid.
-	if !floatsClose(records[1].Fee, 0.5) {
-		t.Errorf("records[1].Fee = %v, want 0.5 (current over-attribution behavior)", records[1].Fee)
+	// Second partial close consumes the remaining half of the open trade:
+	// exit fee 0.1 + remaining entry fee 0.2 = 0.3. Total entry fee attributed
+	// across both closes must equal the 0.4 actually paid.
+	if !floatsClose(records[1].Fee, 0.3) {
+		t.Errorf("records[1].Fee = %v, want 0.3", records[1].Fee)
+	}
+	totalEntryFee := records[0].Fee + records[1].Fee - 0.2 // subtract the two exit fees
+	if !floatsClose(totalEntryFee, 0.4) {
+		t.Errorf("total attributed entry fee = %v, want 0.4 (fee actually paid)", totalEntryFee)
 	}
 }