pxe: arch-aware NBP + undionly.kpxe for legacy BIOS clients

Legacy-BIOS PXE clients booting Blancco reported "NBP is too big to
fit in free base memory". Cause: dnsmasq unconditionally served
ipxe.efi (~675KB EFI binary) which legacy BIOS PXE ROMs cannot
execute and which exceeds their NBP cap.

Fix:
- Add undionly.kpxe (~70KB BIOS-mode iPXE, from boot.ipxe.org).
- dnsmasq: dhcp-match on option:client-arch,0 (BIOS) -> undionly.kpxe;
  default (everything else, including UEFI x86_64 arch 7 and 9) keeps
  getting ipxe.efi. Tag form is reversible: if the match fails to
  evaluate, fallback is the working EFI path, not the new binary.
- Ansible TFTP-copy loop: mirror undionly.kpxe alongside ipxe.efi.
- .gitignore exception: track the open-source kpxe binary so the
  air-gapped USB build stays self-contained.

UEFI clients unchanged. Blancco/Clonezilla/WinPE chain after the
iPXE menu is identical regardless of which iPXE variant delivered it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

playbook/pxe_server_setup.yml

@@ -163,7 +163,15 @@
           # dhcp-option=6,8.8.8.8
           enable-tftp
           tftp-root={{ tftp_dir }}
-          dhcp-boot=ipxe.efi
+          # Arch-aware NBP: legacy BIOS PXE ROMs (client-arch=0) cannot run
+          # the EFI iPXE binary and report "NBP is too big to fit in free
+          # base memory" because ipxe.efi (~675KB) exceeds the BIOS PXE
+          # NBP cap. Serve undionly.kpxe (~70KB, BIOS-mode iPXE) to them
+          # instead. Everything else (UEFI x86_64 = arch 7 or 9, plus any
+          # future arches) keeps getting ipxe.efi - default-safe.
+          dhcp-match=set:bios,option:client-arch,0
+          dhcp-boot=tag:bios,undionly.kpxe
+          dhcp-boot=tag:!bios,ipxe.efi
           log-dhcp
           # Per-lease state cleanup: flush conntrack + port-445 sockets for
           # the client IP on add/del. Prevents "System error 53" when a PXE
@@ -727,6 +735,7 @@
         mode: '0755'
       loop:
         - ipxe.efi
+        - undionly.kpxe
       ignore_errors: yes
 
     - name: "Copy boot tool files from USB (Clonezilla, Blancco, Memtest)"