A 58-year-old man with chronic kidney disease stage 4 (eGFR 22 mL/min/1.73m²) presents to the outpatient clinic for routine follow-up. He is asymptomatic. Laboratory investigations reveal: pH 7.32, PaCO₂ 38 mmHg, HCO₃⁻ 18 mEq/L, Na⁺ 138 mEq/L, Cl⁻ 104 mEq/L, K⁺ 5.8 mEq/L. Anion gap calculated as 12 mEq/L. What is the primary acid-base disorder and the appropriate secondary respiratory response?