A 32-year-old man from Mumbai presents with a 3-week history of fever, night sweats, and progressive dyspnea. He is known to be HIV-positive with a CD4 count of 85 cells/μL (measured 2 weeks ago). On examination, he has bilateral crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. Arterial blood gas shows PaO₂ 65 mmHg on room air. He has not been on antiretroviral therapy (ART) or prophylaxis. What is the most likely diagnosis?