Perioperative Tislelizumab Plus Chemotherapy Versus Chemotherapy Alone in MHC-II-Positive Gastric/GEJ Cancer

Inclusion Criteria: * Willingness to participate and signed informed consent form * ≥18 years old * Histologically confirmed gastric adenocarcinoma or gastroesophageal junction (GEJ) adenocarcinoma * MHC-II immunohistochemistry (IHC) 2+/3+ * Locally advanced disease (cT3-4a, N+, M0) confirmed by CT and/or diagnostic laparoscopy (AJCC 8th edition) * No previous anticancer therapy (surgery, radiotherapy, chemotherapy, targeted therapy, immunotherapy, etc.) * Scheduled to undergo curative resection after neoadjuvant therapy * Ability to swallow oral medication * ECOG performance status 0-1 * Estimated survival ≥6 months * Hematological (without transfusion/G-CSF support within 14 days):ANC ≥1.5×10⁹/, Platelets ≥80×10⁹/L, Hemoglobin ≥80 g/L. Hepatic/Renal: Total bilirubin \<1.5×ULN, ALT/AST ≤2.5×ULN, Serum creatinine ≤1.5×ULN or CrCl \>50 mL/min (calculated by Cockcroft-Gault formula: Male: CrCl = \[(140-age) × weight (kg)\] / (72 × serum Cr \[mg/dL\]), Female: CrCl = \[(140-age) × weight (kg)\] / (72 × serum Cr \[mg/dL\]) × 0.85. * Contraception Requirements: Female participants of childbearing potential: Negative serum pregnancy test within 7 days before enrollment; agreement to use highly effective contraception during treatment and for 120 days after last dose. Female participants of childbearing potential: Negative serum pregnancy test within 7 days before enrollment; agreement to use highly effective contraception during treatment and for 120 days after last dose. Exclusion Criteria: * Tumors deemed unresectable due to disease extent, surgical contraindications, or patient refusal. * Known microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumors. * History of or concurrent other malignancies (except adequately treated non-melanoma skin cancer or carcinoma in situ). * Chronic or clinically significant conditions that may compromise treatment tolerance (e.g., severe cardiac disease, uncontrolled hypertension, significant hepatic/renal dysfunction). * History of gastrointestinal perforation, intra-abdominal abscess, or bowel obstruction within 3 months (or clinical/radiologic suspicion of obstruction). * Presence of active ulcers, non-healing wounds, or fractures. * Arterial/venous thrombosis within 6 months (e.g., stroke, transient ischemic attack, deep vein thrombosis, pulmonary embolism). * Urinalysis showing ≥++ protein with confirmed 24-hour urine protein \>1.0 g. * Requiring systemic antibiotics, antivirals, or antifungals. * Hepatitis B: HBsAg-positive with HBV DNA ≥500 IU/mL. Hepatitis C: HCV antibody-positive with HCV RNA above ULN. * Congenital or acquired (e.g., HIV infection). * Active autoimmune disease or history of autoimmune disease with relapse potential. * Prior or planned organ/allogeneic bone marrow transplantation. * Interstitial lung disease (ILD), history of steroid-treated ILD, active pneumonia on screening CT, or active tuberculosis. * Current or recent use of immunosuppressants or systemic corticosteroids (except physiologic replacement doses). * Received live attenuated vaccines within 28 days before treatment or requiring them during/within 60 days post-treatment. * Known allergy to any study drug or excipients. * Currently breastfeeding. * Any condition that, per investigator judgment, may jeopardize patient safety or study completion.