A 68-year-old man with COPD (GOLD Stage 3, FEV1 42% predicted) and prior CABG was admitted with severe community-acquired pneumonia progressing to ARDS requiring mechanical ventilation. After 8 days, his ARDS has resolved. He passed a 30-minute spontaneous breathing trial this morning with normal vital signs, RSBI 82, and stable gas exchange. He was extubated 6 hours ago but now presents with progressive tachypnea (RR 34), weak cough, accessory muscle use, and diaphoresis. SpO2 is 89% on 6 L nasal cannula. ABG shows pH 7.31, PaCO2 55 (up from 42 pre-extubation), PaO2 62. Bedside spirometry reveals the pattern marked **B** in the diagram: FVC 8 mL/kg, MIP -18 cmH2O, and peak cough flow 130 L/min. Which of the following is the most appropriate next step in management?