A 58-year-old Indian man presents with progressive dysphagia and early satiety for 3 months. He has lost 8 kg in weight. On examination, he appears cachectic with a palpable epigastric mass. Upper endoscopy reveals a large ulcerated lesion in the gastric antrum with rolled edges and a necrotic base. Biopsy shows poorly differentiated adenocarcinoma with signet-ring cells. CT abdomen shows thickening of the gastric wall with involvement of the lesser curvature and perigastric lymph nodes. Which histological type carries the worst prognosis in this patient's presentation?