A 52-year-old man with chronic kidney disease (eGFR 18 mL/min/1.73 m²) presents with dyspnea and confusion. He reports poor appetite and nausea for 2 weeks. Vital signs: RR 28/min, BP 145/92 mmHg. Arterial blood gas: pH 7.28, PaCO₂ 32 mmHg, HCO₃⁻ 14 mEq/L, anion gap 18 mEq/L. Serum creatinine 4.2 mg/dL, K⁺ 6.1 mEq/L. What is the acid-base diagnosis?