Adds modules/pxe-boot/mount-pxe-images.nix, which mounts
server.sweet.home:/tank/proxmox/pxe-images at /mnt/pxe-images via NFSv4.2
(x-systemd.automount on Proxmox VMs, nofail on LXC containers — same pattern
as docker/mount-data.nix). The pxe-boot build-type now imports this module and
replaces the previous local /srv/pxe/http/images directory rule with an L+
symlink pointing to /mnt/pxe-images, so large images (ISOs, disk images) live
on the NFS share rather than the host's own root disk.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
