A 72-year-old woman from Bangalore presents with a 3-year history of progressive lymphadenopathy and fatigue. She denies fever or night sweats. Physical examination reveals generalized lymphadenopathy (cervical, axillary, inguinal) and mild splenomegaly. Complete blood count: WBC 18,500/μL (85% small mature-appearing lymphocytes, 10% prolymphocytes, 5% smudge cells), Hb 9.8 g/dL, platelets 95,000/μL. Flow cytometry shows CD5+, CD19+, CD23+ B cells. Bone marrow biopsy shows lymphoid infiltration (40% lymphocytes). What is the most likely diagnosis?