A 58-year-old man from rural India presents with progressive dysphagia and epigastric pain for 4 months. He reports early satiety and unintentional weight loss of 8 kg. On examination, he appears cachectic. Upper endoscopy reveals a large, ulcerated lesion in the antrum with rolled edges and surrounding erythema. Biopsy shows adenocarcinoma with signet-ring cells. CT abdomen shows thickening of the gastric wall with involvement of regional lymph nodes but no distant metastases. Which histological subtype carries the worst prognosis in this patient?