aws-subnet-ip-exhaustion — know when a VPC subnet runs out of IPs before the ENI allocation fails

Forecast when each VPC subnet runs out of free IPs. Walks every subnet in an account or region, counts what is actually consuming addresses (ENIs, not just instances), compares free capacity against the CIDR size, and projects a days-to-exhaustion number from a recorded history file so the warning arrives before the ENI allocation fails.