MSS-type Low Rectal Cancer，Neoadjuvant Chemoradiotherapy, QL1706

he goal of this prospective, exploratory clinical trial is to learn if neoadjuvant chemoradiotherapy combined with QL1706 (a dual PD-1/CTLA-4 antibody) can effectively treat in patients with MSS-type low rectal cancer (tumor lower edge ≤ 7cm from the anal verge, aged 18-75). The main questions it aims to answer are: What is the complete response (CR) rate, including clinical complete response (cCR) and pathological complete response (pCR)? What are the 3-year and 5-year overall survival (OS) and disease-free survival (DFS) rates? Researchers will compare two cohorts of patients based on tumor stage (Cohort 1: early low rectal cancer cT1-3N0M0; Cohort 2: locally advanced low rectal cancer cT4N0M0 or cTanyN+M0) to see if the combination treatment yields different efficacy and safety outcomes depending on the disease stage. Participants will: Receive either short-course radiotherapy (25Gy/5 fractions) or long-course radiotherapy (50Gy/25 fractions) based on their assigned cohort. Receive 4 cycles of chemotherapy (Capeox: Capecitabine + Oxaliplatin) combined with QL1706. Undergo assessments including pelvic MRI, endorectal ultrasound, and digital rectal exams to evaluate tumor regression. Based on the tumor response, undergo surgery (radical resection or local excision) or enter a Watch \& Wait follow-up protocol. Provide blood, urine, stool, and tissue samples for biomarker and exploratory analyses.