A 36-year-old man with poorly controlled HIV (CD4 count 70/mm3) presents with 3 months of progressively increasing skin lesions in the groin and proximal medial thighs. He reports no fever or recent travel. Examination shows multiple non-tender, violaceous papules and plaques in the inguinal region. A punch biopsy demonstrates spindle cell proliferation forming slit-like vascular spaces with an infiltrate rich in lymphocytes and plasma cells. Which of the following is the most likely diagnosis?