A Prospective, Single-Arm, Single-Center Phase II Study Evaluating the Efficacy and Safety of Chidamide Combined With PD-L1 Inhibitor, Carboplatin, and Etoposide as First-Line Treatment in Patients With Extensive-Stage Small-Cell Lung Cancer (ES-SCLC)

Inclusion Criteria: * Age ≥18 and ≤75 years. * Histologically confirmed ES-SCLC, unsuitable for local radical therapy. * No prior systemic therapy for ES-SCLC. * ECOG performance status 0-1. * At least one measurable lesion per RECIST v1.1. * Expected survival ≥3 months. * Adequate organ function (hematological, hepatic, renal, cardiac). * Effective contraception from consent until 180 days after last dose. * For active HBV infection: HBV DNA \<2000 IU/mL within 28 days before treatment and on stable antiviral therapy. * Recovery from prior therapy toxicities to ≤ Grade 1 (except alopecia). * Signed informed consent. Exclusion Criteria: * Factors significantly affecting oral drug absorption. * Prior HDAC inhibitor or immune checkpoint inhibitor therapy. * Known allergy to any study drug component. * Other malignancy within past 5 years (except certain cured cancers). * Participation in another clinical trial within 4 weeks. * Immunodeficiency, HIV positivity, or organ transplant history. * Uncontrolled cardiovascular disease or QTc \>450 ms. * Pregnancy, lactation, or unwillingness to use effective contraception. * Other severe comorbid conditions deemed unsafe by investigator. * History of neurological or psychiatric disorders. * Any condition making the patient unsuitable per investigator judgment.