A 62-year-old woman undergoes curative gastrectomy for gastric adenocarcinoma (pT3N2M0). Histopathology shows intestinal-type gastric cancer with signet-ring cell component. The surgical specimen shows a 5 cm lesion with invasion into the muscularis propria and perigastric lymph node involvement. Which investigation is most appropriate to assess for occult peritoneal metastases before adjuvant chemotherapy planning?