feat(deploy): add PXE controller role for multi-node netboot

[KerwinTsaiii]

Summary

• Adds a pxe_controller Ansible role and pb-pxe-controller.yml playbook that turn one service machine into a PXE controller so diskless agents can netboot and auto-join a K3s cluster.
• The role sets up dnsmasq (Proxy-DHCP + TFTP), a debootstrap NFS rootfs under /srv/nfs/rootfs, BIOS/UEFI boot menus in /srv/tftp, and an Apache /k3s/ endpoint for join credentials.
• Netboot agents auto-join via a baked-in k3s-auto-join.service; local-disk persistence keeps the K3s node identity across reboots.

Design notes

• Site-specific networking values (pxe_network_interface, pxe_subnet, pxe_dns_servers, pxe_controller_ip, pxe_k3s_server_ips) are empty by default and asserted at run time, so no site IPs ship in the repo.
• The agent K3s version is pinned via pxe_k3s_version (defaults to v1.32.3+k3s1) to match the server.
• No vendor NIC driver and no Cloudflare WARP bits are bundled; agents are expected to have an in-kernel NIC driver (add the module to pxe_initramfs_modules if needed).

Docs

• The end-to-end usage guide is a separate docs-only PR against the docs branch (docs: add 3 Node Mini-Cluster Example (PXE netboot) guide #113).

Test plan

• YAML parses; playbook/role structure validated.
• Run pb-pxe-controller.yml against a real service machine and netboot at least one agent.

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