A 30-year-old male developed shortness of breath after activity 6 months ago, and has experienced shortness of breath at rest for the past 2 months, along with occasional white sputum production, leading to hospitalization. Physical exam: RR 24/min, few crackles audible at lung bases. Chest X-ray: bilateral diffuse 'ground-glass' opacities. Bronchoalveolar lavage fluid appears milky, forms sediment upon standing, has high lipoprotein content, and is PAS-positive. The most likely diagnosis is: