fixed multiplication asymmetry

0-source/Just.PreciseMath/DoubleDouble.Arithmetic.cs

@@ -91,7 +91,10 @@ public readonly partial struct DoubleDouble :
     }
 
     /// <summary>Multiplies expansions using an FMA product residual and cross terms.</summary>
-    /// <remarks>Results are approximate double-double values, not universally correctly rounded.</remarks>
+    /// <remarks>
+    /// Swapping operands preserves both result components bit-for-bit.
+    /// Results are approximate double-double values, not universally correctly rounded.
+    /// </remarks>
     public static DoubleDouble operator *(DoubleDouble left, DoubleDouble right)
     {
         if (!IsFinite(left) || !IsFinite(right) || left._high == 0.0 || right._high == 0.0)
@@ -103,6 +106,14 @@ public readonly partial struct DoubleDouble :
             return PreciseMathHelper.MultiplyBoundary(left, right);
         }
 
+        // Sequential cross-term FMAs round in order. Use the same operand order
+        // in both directions, including when highs tie. Inputs here are finite
+        // and nonzero with canonical lows; the exact boundary path is symmetric.
+        if (left._high > right._high || (left._high == right._high && left._low > right._low))
+        {
+            (left, right) = (right, left);
+        }
+
         (double product, double error) = PreciseMathHelper.TwoMultiply(left._high, right._high);
         error = Math.FusedMultiplyAdd(left._high, right._low, error);
         error = Math.FusedMultiplyAdd(left._low, right._high, error);