perf(solvers): scope default one-equation calls to one thread

src/fesa/solvers/linear/mkl_pardiso_tangent_solver.cpp

@@ -13,6 +13,7 @@
 
 #include "fesa/math/sparse_matrix.h"
 #include "fesa/math/vector.h"
+#include "math/dense_blas_internal.h"
 
 namespace fesa {
 namespace {
@@ -230,6 +231,8 @@ class MklPardisoTangentSolver::Impl {
   /// @brief Calls PARDISO with retained private arrays and phase state.
   void CallPardiso(const MKL_INT phase, double* rhs, double* correction,
                    MKL_INT& error) {
+    const dense_blas_internal::ScopedSingleEquationThreads thread_scope{
+        static_cast<std::size_t>(equation_count_)};
     pardiso(pt_.data(), &max_factorizations_, &matrix_number_, &mtype_, &phase,
             &equation_count_, values_.data(), row_offsets_.data(),
             column_indices_.data(), permutation_.data(), &rhs_count_,