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

src/fesa/math/dense_blas_internal.h

@@ -11,6 +11,21 @@
 
 namespace fesa::dense_blas_internal {
 
+/// @brief Limits only default-policy one-equation backend calls to one thread.
+/// @note Existing caller TLS overrides are retained. Destruction restores the
+/// default request; process-wide MKL/OpenMP settings are never changed.
+class ScopedSingleEquationThreads {
+ public:
+  explicit ScopedSingleEquationThreads(std::size_t equation_count) noexcept;
+  ScopedSingleEquationThreads(const ScopedSingleEquationThreads&) = delete;
+  ScopedSingleEquationThreads& operator=(const ScopedSingleEquationThreads&) =
+      delete;
+  ~ScopedSingleEquationThreads();
+
+ private:
+  bool restore_default_{false};
+};
+
 /// @brief Converts a dense storage length to the private MKL integer type.
 /// @return The converted length or an uncategorized overflow failure.
 Result<MKL_INT> ToMklSize(std::size_t size);