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

src/fesa/math/dense_blas_internal.cpp

@@ -8,6 +8,24 @@
 
 namespace fesa::dense_blas_internal {
 
+ScopedSingleEquationThreads::ScopedSingleEquationThreads(
+    const std::size_t equation_count) noexcept {
+  // The measured 1x1 PARDISO case has backend/thread overhead and an
+  // intermittently failing default-thread lifecycle. Broader sizes are not
+  // inferred from this auxiliary evidence.
+  if (equation_count != 1U) return;
+  const int previous = mkl_set_num_threads_local(1);
+  if (previous == 0) {
+    restore_default_ = true;
+  } else {
+    static_cast<void>(mkl_set_num_threads_local(previous));
+  }
+}
+
+ScopedSingleEquationThreads::~ScopedSingleEquationThreads() {
+  if (restore_default_) static_cast<void>(mkl_set_num_threads_local(0));
+}
+
 bool CholeskyLowerAndInverse(const Matrix& matrix, Matrix& lower,
                             Matrix& inverse_lower) {
   if (matrix.Rows() != matrix.Columns())