A 34-year-old man presents with a 2-week history of a dry cough and increasing shortness of breath on exertion. He is known to be HIV positive but has been non-adherent with his antiretroviral therapy for the past year. His CD4 count 6 months ago was 150 cells/mm³. On examination, he is tachypnoeic (RR 28/min), afebrile, and his oxygen saturation is 90% on room air. Chest X-ray shows bilateral diffuse interstitial infiltrates. What is the most likely opportunistic infection?