Integrate Refactor solver sources and dual MSVC CMake builds

src/fesa/math/high_low_arithmetic.h

@@ -88,6 +88,12 @@ inline double Rounded(HighLow value) {
 /// @brief Adds normalized expansions; rounding of discarded tail is bounded.
 inline HighLow Add(HighLow left, HighLow right) {
   if (!IsNormalized(left) || !IsNormalized(right)) return InvalidHighLow();
+  // Exact-zero additions only normalize signed zero in the retained pair.
+  // The nonzero graph and its discarded-tail association stay unchanged.
+  if (left.high == 0 && left.low == 0)
+    return {right.high, right.low == 0 ? 0.0 : right.low};
+  if (right.high == 0 && right.low == 0)
+    return {left.high, left.low == 0 ? 0.0 : left.low};
   const auto head = TwoSum(left.high, right.high);
   const auto tail = TwoSum(left.low, right.low);
   const auto middle = TwoSum(head.low, tail.high);
@@ -164,9 +170,16 @@ inline HighLow TwoProduct(double left, double right) {
 inline HighLow Multiply(HighLow left, HighLow right) {
   if (!IsNormalized(left) || !IsNormalized(right)) return InvalidHighLow();
   auto product = TwoProduct(left.high, right.high);
-  product = Add(product, TwoProduct(left.high, right.low));
-  product = Add(product, TwoProduct(left.low, right.high));
-  return Add(product, TwoProduct(left.low, right.low));
+  // Omitted terms are exact zero; retain the zero normalization that preceded
+  // a nonzero low-high term and the final canonicalization/invalid check.
+  if (right.low != 0)
+    product = Add(product, TwoProduct(left.high, right.low));
+  else if (left.low != 0)
+    product = Add(product, HighLow{});
+  if (left.low != 0)
+    product = Add(product, TwoProduct(left.low, right.high));
+  return Add(product, left.low != 0 && right.low != 0
+                          ? TwoProduct(left.low, right.low) : HighLow{});
 }
 /// @brief Orders canonical values; 2 means unordered/unsupported input.
 inline int Compare(HighLow left, HighLow right) {