A 39-year-old man with a 12-year history of HIV infection is brought to clinic by his partner due to 4 months of progressive behavioral changes. He has become socially withdrawn, has episodes of inappropriate laughing, and has developed bizarre beliefs that coworkers are "sending messages" through the television. His partner also reports increasing difficulty with attention and mental calculations; he frequently loses track of conversations and cannot manage basic finances. He has been intermittently adherent to antiretroviral therapy. Vitals are normal. Neurologic exam shows slowed gait and mild bradykinesia without focal weakness or sensory loss. Laboratory studies show CD4 count of 110 cells/mm³ and HIV viral load of 320,000 copies/mL. Brain MRI demonstrates a diffuse increase in T2/FLAIR signal intensity within the cerebral white matter without a discrete mass lesion.

Which of the following is the most likely diagnosis?