Integrate Refactor solver sources and dual MSVC CMake builds

src/fesa/math/compensated_sum.h

@@ -0,0 +1,30 @@
+#ifndef FESA_MATH_COMPENSATED_SUM_H_
+#define FESA_MATH_COMPENSATED_SUM_H_
+
+#include <cmath>
+
+namespace fesa::math_internal {
+
+/// @brief Preserves the compensated scalar accumulation graph and caller order.
+inline void AddCompensated(const double term, double* sum, double* correction) {
+  const double next = *sum + term;
+  *correction += std::abs(*sum) >= std::abs(term)
+                     ? (*sum - next) + term
+                     : (term - next) + *sum;
+  *sum = next;
+}
+
+/// @brief Owns one sum and its correction without changing reduction order.
+class CompensatedSum {
+ public:
+  void Add(const double term) { AddCompensated(term, &sum_, &correction_); }
+  double Value() const { return sum_ + correction_; }
+
+ private:
+  double sum_ = 0.0;
+  double correction_ = 0.0;
+};
+
+}  // namespace fesa::math_internal
+
+#endif  // FESA_MATH_COMPENSATED_SUM_H_