Preserve -0 in integer multiplication fast paths

Jint.Tests/Runtime/NumberTests.cs

@@ -221,4 +221,39 @@ public void CompoundAssignmentMatchesBinaryOperatorSemantics()
         var tdzUpdate = Assert.Throws<Jint.Runtime.JavaScriptException>(() => engine.Evaluate("(function() { { y++; let y; } })()"));
         Assert.Equal("ReferenceError", tdzUpdate.Error.AsObject().Get("constructor").AsObject().Get("name").AsString());
     }
+
+    [Fact]
+    public void IntegerMultiplicationPreservesNegativeZero()
+    {
+        var engine = new Engine();
+        // Number::multiply: a zero product takes a negative sign when the operand signs differ.
+        // The integer fast paths (binary * and compound *=) cannot represent -0 and must route
+        // zero products through double arithmetic; test every operand shape the lanes serve.
+        var result = engine.Evaluate("""
+            (function () {
+                var zero = 0, negFive = -5, five = 5;
+                var r = [];
+                r.push(Object.is(zero * negFive, -0));
+                r.push(Object.is(negFive * zero, -0));
+                r.push(Object.is(zero * five, 0));
+                r.push(Object.is(zero * zero, 0));
+                r.push(Object.is(0 * -5, -0));
+                var arr = [0, -5];
+                r.push(Object.is(arr[0] * arr[1], -0));
+                var o = { a: 0, b: -5 };
+                r.push(Object.is(o.a * o.b, -0));
+                function f(x, y) { return x * y; }
+                r.push(Object.is(f(0, -5), -0));
+                var acc = 0; acc *= -5;
+                r.push(Object.is(acc, -0));
+                var acc2 = -5; acc2 *= 0;
+                r.push(Object.is(acc2, -0));
+                var acc3 = 0; acc3 *= 5;
+                r.push(Object.is(acc3, 0));
+                return r.join(',');
+            })()
+            """).AsString();
+
+        Assert.Equal("true,true,true,true,true,true,true,true,true,true,true", result);
+    }
 }

Jint/Runtime/Interpreter/Expressions/JintAssignmentExpression.cs

@@ -281,7 +281,12 @@ private JsValue ComputeCompound(EvaluationContext context, JsValue originalLeftV
                 {
                     if (AreIntegerOperands(originalLeftValue, rval))
                     {
-                        return JsNumber.Create((long) originalLeftValue.AsInteger() * rval.AsInteger());
+                        var product = (long) originalLeftValue.AsInteger() * rval.AsInteger();
+                        // Number::multiply gives -0 for zero products of opposite signs (0 * -5),
+                        // which integer math cannot represent — route through double arithmetic
+                        return product != 0
+                            ? JsNumber.Create(product)
+                            : JsNumber.Create((double) originalLeftValue.AsInteger() * rval.AsInteger());
                     }
 
                     var leftNumeric = TypeConverter.ToNumeric(originalLeftValue);

Jint/Runtime/Interpreter/Expressions/JintBinaryExpression.cs

@@ -1162,7 +1162,12 @@ protected override object EvaluateInternal(EvaluationContext context)
             }
             else if (AreIntegerOperands(left, right))
             {
-                result = JsNumber.Create((long) left.AsInteger() * right.AsInteger());
+                var product = (long) left.AsInteger() * right.AsInteger();
+                // Number::multiply gives -0 for zero products of opposite signs (0 * -5), which
+                // integer math cannot represent — route zero products through double arithmetic
+                result = product != 0
+                    ? JsNumber.Create(product)
+                    : JsNumber.Create((double) left.AsInteger() * right.AsInteger());
             }
             else if (left._type == InternalTypes.Number && right._type == InternalTypes.Number)
             {