Pods stuck pending during scale-up

During a planned product launch your service tries to scale from 80 to 200 pods. The HPA requests the replicas, but new pods get stuck in Pending and the cluster-autoscaler logs show it adding nodes, yet only ~30 of the 120 new pods ever schedule. Latency and error rate climb because the fleet is stuck at ~110 effective replicas. Node group capacity is nowhere near its max. Digging in, the cluster events show FailedScheduling: 0/x nodes available: insufficient IPs on the pending pods and the VPC subnet's available-IP gauge reads near zero. The autoscaler keeps trying. How do you triage and mitigate the launch, and what's the durable fix?