A 31-year-old woman presents with pallor, gingival bleeding, and menorrhagia for 6 months. Laboratory tests show pancytopenia in peripheral blood, absolute reticulocyte count of 15×10<sup>9</sup>/L. Bone marrow aspiration from the anterior and posterior iliac spines reveals hemodilution, while sternal puncture shows active nucleated cell proliferation dominated by late normoblasts and mature lymphocytes, with no megakaryocytes seen on the slide. The most likely diagnosis is: