A 52-year-old man with a 30-year smoking history and COPD presents with a 3-day history of productive cough and fever. His chest X-ray shows a left lower lobe infiltrate consistent with pneumonia. Arterial blood gas on room air: pH 7.32, PaCO₂ 58 mmHg, PaO₂ 62 mmHg, HCO₃⁻ 28 mEq/L. His hemoglobin is 18 g/dL (polycythemia from chronic hypoxia). Despite the low PaO₂, his oxygen saturation is 88%. Which physiologic adaptation has occurred to maintain tissue oxygen delivery in this chronic hypoxic state?