A 40-year-old man presents with a 2-week history of persistent dry cough, worsening shortness of breath on exertion, fever, and malaise. He is HIV positive with a CD4 count of 150 cells/mm³ and not currently on antiretroviral therapy. His chest X-ray shows bilateral interstitial infiltrates. Oxygen saturation is 90% on air. What is the most likely opportunistic infection?