perf(math): route ordinary products through MKL and preserve arithmetic

include/fesa/math/sparse_matrix.h

@@ -46,8 +46,12 @@ class SparseMatrix {
   /// @brief Returns CSR values including preserved structural zeros.
   const std::vector<double>& Values() const noexcept;
 
-  /// @brief Multiplies this matrix by a dense vector in stable CSR order.
-  /// @throws std::invalid_argument if the dimensions are incompatible.
+  /// @brief Multiplies canonical CSR by a dense vector through private MKL.
+  /// @throws std::invalid_argument if dimensions or CSR 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.
   Vector Multiply(const Vector& rhs) const;
 
   /// @brief Validates shape, indices, ordering, and finite CSR values.