A 72-year-old woman from Bangalore is referred to hematology for evaluation of a 2-year history of progressive lymphocytosis. She reports no B symptoms. Physical examination reveals mild lymphadenopathy in the cervical and axillary regions. CBC: Hb 11.8 g/dL, WBC 28,000/μL (lymphocytes 78%), platelets 95,000/μL. Peripheral blood smear shows small, mature-appearing lymphocytes with scant cytoplasm and a characteristic 'smudge cell' appearance. Flow cytometry: CD5+, CD19+, CD23+, surface immunoglobulin (weak), CD10 negative. Serum LDH is normal. What is the most likely diagnosis?