A 62-year-old man presents with a 3-month history of a persistent, non-productive cough and progressive shortness of breath on exertion. He is a retired stone mason. On examination, there are fine, bibasal, end-inspiratory crackles. A chest X-ray shows bilateral reticulonodular shadowing, predominantly in the upper zones. Pulmonary function tests show a restrictive pattern. What is the most likely diagnosis?