A 60-year-old man presents with gradually worsening breathlessness on exertion over the past year, accompanied by a persistent dry cough. He is a lifelong non-smoker but worked as a stonemason for 30 years. Examination reveals fine bibasal end-inspiratory crackles. Chest X-ray shows bilateral reticular opacities, predominantly in the lower zones. Pulmonary function tests show FEV1 75% predicted, FVC 60% predicted, FEV1/FVC ratio 92%. What is the most likely diagnosis?