A 58-year-old man from Delhi presents with a 3-month history of altered bowel habits, tenesmus, and blood-stained stools. Digital rectal examination reveals a hard, irregular mass 6 cm from the anal verge. Colonoscopy confirms a stenosing lesion of the rectosigmoid with biopsy-proven adenocarcinoma. CT staging shows a T3N1M0 tumour with no distant metastases. The patient is fit for surgery with no significant comorbidities. What is the most appropriate next step in management?