A 28-year-old woman with a 12-year history of focal seizures presents to the epilepsy clinic. Her seizures are characterized by an initial rising epigastric sensation, followed by behavioral arrest with lip smacking and hand fumbling, and postictal confusion lasting 10–15 minutes. Brain MRI shows left hippocampal atrophy with increased T2/FLAIR signal. Her interictal EEG is shown in the diagram. The structure marked **A** represents focal anterior temporal spike-wave activity maximal at F7/T3. She has failed trials of levetiracetam, lamotrigine, and oxcarbazepine over the past 3 years. Which of the following is the most appropriate next step in management?