Flesh out the single-public-IP pf setup with what actually worked

Replaces the sketch (bare ifconfig alias, isolated nat line) with what we
verified end-to-end on a real VM: the alias persisted via rc.conf, a full
pf.conf modeled on a plain host's firewall, and the pfctl/kldload steps
needed to load and check it safely.

Also documents a real bug hit along the way: `nat ... -> ($ext_if)` is
ambiguous once the interface carries more than one address (the public
IP and the private jail range both live on it) - pf can silently NAT
through the wrong one. The public IP needs to be written explicitly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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