DOCFIX-146: opnsense-edge module + nano-image/config-ISO prep scripts ...

Implements the mechanism identified in today's OPNsense deployment research
(cloud-init confirmed unreliable on FreeBSD; the real native mechanism is
OPNsense's Configuration Importer, which scans an attached volume for
/conf/config.xml and gained ISO9660 support specifically for VM/cloud
automation).

scripts/opnsense-prep-image.sh: downloads the nano image, decompresses,
converts raw->qcow2, resizes -- work create.content.url almost certainly
can't do itself (a plain fetch, no decompression). No hardcoded mirror URL;
OPNSENSE_MIRROR_BASE is required explicitly.

scripts/opnsense-build-config-iso.sh: builds the ISO9660 image containing
/conf/config.xml that the Configuration Importer picks up.

opentofu/modules/opnsense-edge: wires both together, mechanically identical
to modules/cloudinit-vm's cdrom-attach shape but with no libvirt_cloudinit_disk
resource (wrong format for this). Explicit lan_network_name/wan_network_name
variables rather than an ordered list, since OPNsense's interface-1-is-LAN
convention makes list-ordering a real place for a silent mistake to hide. No
PXE boot-order attribute used -- this VM boots from its own disk, not the
network, so node-vm's flagged-unverified boot field doesn't apply here.

Both new scripts' harnesses (4/4, 3/3) exercise REAL missing-tool guards --
qemu-img/genisoimage/xorriso are all genuinely absent from this environment.
Two things flagged, not presented as fact: whether the Importer's ISO9660
behavior holds on a real boot, and whether interface list-order reliably
maps to LAN/WAN NIC enumeration for this guest. config_iso_path has no
default -- real config.xml content per site is still undesigned. Not
instantiated in root main.tf.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A