A patient with HIV and a CD4 count of 120 cells/mm³ presents with a blue-purple vascular lesion on the hard palate. What is the most likely diagnosis?

Kaposi sarcoma (caused by HHV-8) is an AIDS-defining vascular neoplasm that classically presents as a blue, purple, or red macule/nodule on the hard palate in patients with advanced HIV disease (CD4 <200 is the AIDS-defining threshold). A pleomorphic adenoma is a firm, painless salivary gland tumor without this color, and a mucocele/pyogenic granuloma would not match this immunosuppressed presentation pattern.