A 62-year-old woman from Delhi presents with a 2-month history of progressive confusion, memory loss, and personality changes. She has also developed jerky involuntary movements and gait ataxia. MRI brain shows cerebellar atrophy. CSF analysis reveals mild lymphocytic pleocytosis. Serum and CSF anti-Hu antibodies are positive. Subsequent imaging reveals a small-cell lung cancer in the right upper lobe. What is the most likely paraneoplastic neurological syndrome?