Phase II Study of Trastuzumab Rezetecan Combined With Retlirafusp Alfa With or Without CAPOX as Perioperative Therapy for Resectable Locally Advanced Gastric/Gastroesophageal Junction Adenocarcinoma

Inclusion Criteria: 1. Voluntarily sign written informed consent before any study-related procedures 2. Age ≥18 and ≤75 years old 3. Histopathologically confirmed gastric or gastroesophageal junction adenocarcinoma 4. AJCC 8th edition stage II-III resectable disease (cT1-2N+M0, T3-4aNanyM0); negative peritoneal cytology (CY0) confirmed by pre-study laparoscopy 5. HER2 expression status: High HER2 (IHC 3+ or IHC 2+ FISH-positive) OR Low/Intermediate HER2 (IHC 2+ FISH-negative / IHC 1+) 6. No prior systemic anti-tumor therapy (chemotherapy, targeted therapy, immunotherapy, radiotherapy, radical gastrectomy) 7. Planned radical D2 gastrectomy after neoadjuvant treatment completion 8. Able to swallow oral capecitabine tablets intact 9. ECOG Performance Status 0 or 1 10. Estimated overall survival ≥12 months 11. Adequate organ function (no blood products/G-CSF within prior 14 days): * ANC ≥1.5×10⁹/L; PLT ≥80×10⁹/L; Hb ≥90 g/L * Total bilirubin \<1.5×ULN; ALT/AST ≤2.5×ULN * Serum Cr ≤1.5×ULN or CrCl \>50 mL/min * INR, PT, APTT ≤1.5×ULN 12. Fertile female subjects: Negative serum pregnancy test within 72 hours before first dose; all fertile participants agree to effective contraception during treatment and required post-treatment contraceptive window Exclusion Criteria: 1. Siewert type I GEJ tumor or Siewert II tumor requiring cervicothoracic surgical approach 2. Confirmed peritoneal metastasis, positive peritoneal cytology (CY1), or AJCC T4b disease 3. Tumor unresectable or absolute contraindication to radical gastrectomy 4. History of other malignant tumor within past 5 years (excluding cured basal cell skin carcinoma, cervical/breast carcinoma in situ) 5. Uncontrolled hypertension (SBP≥160 mmHg or DBP≥100 mmHg despite optimal medical management) 6. Cardiac dysfunction: NYHA grade ≥2 heart failure, LVEF \<50%, unstable angina, myocardial infarction within 1 year, prolonged QTc interval (male \>450ms, female \>470ms), family history of sudden cardiac death \<40 years old 7. GI perforation/fistula within 6 months, intestinal obstruction within 3 months (not fully resolved) 8. Active severe bleeding disorder or active peptic ulcer disease 9. Arterial/venous thromboembolic event within past 6 months (stroke, DVT, pulmonary embolism etc.) 10. Severe uncontrolled infection (≥CTCAE grade 2) within 4 weeks prior to first dose 11. Active viral hepatitis (HBV DNA ≥2000 IU/mL; active HCV viremia) or HIV positive 12. History or active interstitial lung disease, pulmonary fibrosis, active tuberculosis 13. Active autoimmune disease requiring long-term systemic immunosuppressive therapy 14. Systemic corticosteroid dose \>10 mg prednisone equivalent within 7 days pre-treatment 15. Received live attenuated vaccine within 28 days before enrollment 16. Hypersensitivity to any study drug or excipients 17. Participation in other interventional clinical trials within 4 weeks prior to first dose 18. Lactating female subjects 19. Any medical, psychiatric or social condition judged by investigator to interfere with study compliance or subject safety