fix(math): preserve certified sums and complete MKL algebra

include/fesa/math/sparse_matrix.h

@@ -47,11 +47,11 @@ class SparseMatrix {
   const std::vector<double>& Values() const noexcept;
 
   /// @brief Multiplies canonical CSR by a dense vector through private MKL.
-  /// @throws std::invalid_argument if dimensions or CSR are invalid.
+  /// @throws std::invalid_argument if dimensions, CSR, or RHS are invalid.
   /// @throws std::length_error if a size exceeds the backend integer range.
   /// @throws std::runtime_error if the sparse backend fails.
   /// @note Calls retain no handle; concurrent const calls own their resources.
-  /// Nonfinite products retain IEEE values for consumer finite validation.
+  /// Finite-input overflow is retained for consumer output finite validation.
   Vector Multiply(const Vector& rhs) const;
 
   /// @brief Validates shape, indices, ordering, and finite CSR values.