A 50-year-old male presents with pallor and fatigue for 1 year, accompanied by left upper abdominal discomfort. Physical examination reveals liver 2 cm below the costal margin and spleen palpable at the umbilicus. Lab results: Hb 65 g/L, platelets 160×10⁹/L, peripheral blood shows few immature granulocytes and nucleated red blood cells (RBC). Multiple bone marrow aspirations yield 'dry tap'. The most likely diagnosis is: