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

tests/unit/solvers/linear/tangent_solver_test.cpp

@@ -3,6 +3,7 @@
 #include <gtest/gtest.h>
 
 #include <cstddef>
+#include <future>
 #include <initializer_list>
 #include <limits>
 #include <type_traits>
@@ -13,6 +14,7 @@
 #include "fesa/math/sparse_matrix.h"
 #include "fesa/math/vector.h"
 #include "fesa/solvers/linear/tangent_solver.h"
+#include "math/dense_blas_internal.h"
 
 namespace {
 
@@ -169,4 +171,38 @@ TEST(TangentSolver, ReusesRhsButFreshValuesAndFailureCannotReuseStaleState) {
   EXPECT_TRUE(solver.SolveCorrection(MakeVector({10, -7}), correction).IsOk());
 }
 
+TEST(TangentSolver, SingleEquationScopeRestoresTlsAcrossFreshFailureAndReuse) {
+  const auto worker = [] {
+    const int previous = mkl_set_num_threads_local(0);
+    struct Restore {
+      int previous;
+      ~Restore() { static_cast<void>(mkl_set_num_threads_local(previous)); }
+    } restore{previous};
+    const int default_threads = mkl_get_max_threads();
+    fesa::MklPardisoTangentSolver solver;
+    for (int repeat = 0; repeat < 10; ++repeat) {
+      auto correction = MakeVector({17.0});
+      if (!solver.FactorizeFresh(MakeDenseCsr(1U, {2.0})).IsOk()) return false;
+      if (!solver.SolveCorrection(MakeVector({4.0}), correction).IsOk() ||
+          correction[0U] != 2.0) return false;
+      if (!solver.FactorizeFresh(MakeDenseCsr(1U, {4.0})).IsOk()) return false;
+      if (!solver.SolveCorrection(MakeVector({4.0}), correction).IsOk() ||
+          correction[0U] != 1.0) return false;
+      if (solver.SolveCorrection(MakeVector({
+              std::numeric_limits<double>::infinity()}), correction).IsOk() ||
+          correction[0U] != 1.0) return false;
+      if (solver.FactorizeFresh(MakeDenseCsr(1U, {0.0})).IsOk()) return false;
+      if (solver.SolveCorrection(MakeVector({4.0}), correction).IsOk() ||
+          correction[0U] != 1.0) return false;
+      if (mkl_get_max_threads() != default_threads ||
+          mkl_set_num_threads_local(0) != 0) return false;
+    }
+    return true;
+  };
+  auto first = std::async(std::launch::async, worker);
+  auto second = std::async(std::launch::async, worker);
+  EXPECT_TRUE(first.get());
+  EXPECT_TRUE(second.get());
+}
+
 }  // namespace