A 32-year-old multiparous woman at 40 weeks gestation is in active labor (cervix 6 cm dilated, contractions 4 in 10 minutes, each 50 seconds). Fetal heart rate tracing shows variable decelerations with slow recovery and baseline rate 130 bpm. Maternal vital signs are stable. Which investigation is most appropriate to assess the degree of fetal hypoxia and guide immediate management?