A 60-year-old male presents with a 6-month history of dry cough, fatigue, shortness of breath on exertion, and significant weight loss. Physical examination: respiratory rate 28/min, bilateral end-inspiratory Velcro crackles at lung bases, and clubbing. Chest X-ray: diffuse reticulonodular infiltrates in mid and lower lung fields. Pulmonary function tests show restrictive ventilatory defect and reduced diffusion capacity. The most likely diagnosis is: