A 55-year-old male presents with poor appetite, fatigue, and weight loss for 6 months. Physical exam: dark red facial complexion, liver palpable 1 cm below the costal margin, spleen palpable 5 cm below the costal margin, firm with no tenderness. Laboratory tests: Hb 195g/L, WBC 16.2×10<sup>9</sup>/L, PLT 450×10<sup>9</sup>/L, hematocrit 0.60 (60%). Bone marrow smear shows marked hyperplasia with trilineage proliferation (granulocytic, erythroid, megakaryocytic) and normal cell morphology. A myeloproliferative disorder is diagnosed. The most likely diagnosis is: