Study Comparing Different Neck Treatment Approaches in Patients With Oral Cancer Without Visible Neck Spread

The goal of this clinical trial is to learn whether selective neck dissection involving lymph node levels I-III is as effective as selective neck dissection involving levels I-IV in treating adults with clinically node-negative (cN0) oral squamous cell carcinoma undergoing curative surgery. It will also evaluate whether extending the neck dissection to level IV affects cancer control, surgical complications, shoulder function, and quality of life. The main questions it aims to answer are: * Does selective neck dissection of levels I-III provide regional recurrence-free survival comparable to selective neck dissection of levels I-IV? * Does extending the neck dissection to level IV improve overall survival, disease-free survival, or reduce regional recurrence? * Does selective neck dissection of levels I-III result in less postoperative morbidity, better shoulder function, and improved q