Host port already in use by forwarded_port rule

Goal: Define a config.vm.network :forwarded_port mapping in a Vagrantfile that exposes a guest TCP service on a chosen host port without disrupting an existing host listener.

Constraint: Vagrant regenerates host-level NAT forwarding rules on every vagrant up and vagrant reload, and the mapping remains active for the VM lifetime. Host operating systems differ: Linux uses iptables, Windows uses netsh. Vagrant 2.2 and earlier do not support the guest option, while 2.3.x added it to influence rule syntax.

Uncertainty: It is not documented whether Vagrant validates host-port availability before creating the rule, what happens when another process already binds the port, or how to detect the conflict before the VM starts.

1. Does Vagrant abort VM startup if the configured host port is already in use?
2. Can a Vagrantfile detect an existing host listener and select an alternate host port automatically?
3. Does the guest option change conflict behavior on Vagrant 2.3.x compared with earlier releases?