Radical Surgery Versus Non-Surgery in Gastric Cancer Patients With Peritoneal Metastasis After Successful Conversion Therapy

The goal of this clinical trial is to learn if radical surgery improves prognosis and quality of life compared with continued standard non-surgical treatment in gastric cancer patients with peritoneal metastasis who have achieved successful conversion therapy. The main questions it aims to answer are: * Does radical surgery prolong overall survival in patients who achieve successful conversion therapy? * Does radical surgery provide better or at least comparable quality of life and safety outcomes compared with continued standard treatment? Researchers will compare radical surgery versus continued standard systemic treatment to see whether radical surgery provides superior clinical benefit for patients after successful conversion therapy. Participants will: * Initial laparoscopic exploration confirms CY0P1 or CY1P1, absence of distant organ metastasis, a PCI ≤ 20, and presence or absence of ascites. * Receive conversion therapy (chemotherapy, targeted/immunotherapy, intraperitoneal therapy, etc.) per standard guidelines, tailored to the patient's condition and molecular-pathological profile * If clinical conversion is confirmed, undergo a second laparoscopic exploration to verify successful conversion * If successful conversion is confirmed (peritoneal lesions cleared or markedly regressed with negative biopsy, no ascites or distant metastases, R0-resectable primary tumor, and improved patient condition), be randomized 1:1 to either the surgery group or the standard treatment group * In the surgery group, undergo curative gastrectomy with D2 lymphadenectomy during the second laparoscopy, followed by adjuvant therapy and maintenance treatment * In the standard treatment group, continue systemic therapy without surgery, with maintenance treatment and salvage surgery permitted if clinically indicated * Complete quality of life questionnaires at predefined time points * Be followed up for survival, disease progression, adverse events and medical resource use outcomes