A 58-year-old male farmer from rural Maharashtra presents with a 4-month history of a non-healing ulcer on the left buccal mucosa. He reports pain during mastication and occasional blood-stained saliva. On examination, the ulcer is 2 cm × 1.5 cm with raised, everted margins and indurated base. Regional lymph nodes are palpable and firm. Intraoral examination reveals leukoplakia in the surrounding mucosa. A biopsy confirms squamous cell carcinoma. Which of the following is the most important prognostic factor in oral cavity carcinoma?