feat(pxe-boot): add Debian bookworm minimal netboot menu entry

Adds a `fetch-debian-netboot.service` oneshot that downloads the Debian
bookworm netboot kernel and initrd from deb.debian.org on first boot,
stages them under /srv/pxe/http/debian/, and serves them via a generated
debian.ipxe chain script. The service is idempotent — it skips the
download if both files are already present.

Also merges the previously split systemd.tmpfiles.rules and
systemd.services blocks into a single systemd = { ... } attrset to
satisfy statix's repeated-keys lint.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/pxe-boot.md

@@ -15,6 +15,7 @@ rescue/inspection use.
 - TFTP root for first-stage bootloaders: `/srv/pxe/tftp`
 - iPXE entry script: `/srv/pxe/http/boot.ipxe`
 - Generated iPXE menu: `/srv/pxe/http/menu.ipxe`
+- Debian Minimal iPXE script: `/srv/pxe/http/debian.ipxe`
 - SystemRescue iPXE script: `/srv/pxe/http/systemrescue.ipxe`
 - TFTP fallback script: `/srv/pxe/tftp/autoexec.ipxe`
 - Boot binaries copied from the Nix `ipxe` package:
@@ -31,6 +32,7 @@ The host creates these directories with systemd tmpfiles:
 /srv/pxe/http/images
 /srv/pxe/http/auto-installer
 /srv/pxe/http/nixos-minimal
+/srv/pxe/http/debian
 /srv/pxe/http/systemrescue
 /srv/pxe/http/ubuntu
 /srv/pxe/http/rescue
@@ -54,6 +56,7 @@ The generated menu currently exposes entries for:
 
 - NixOS Auto-Installer
 - NixOS Minimal
+- Debian Minimal
 - SystemRescue environment
 - iPXE shell
 - Reboot
@@ -84,6 +87,29 @@ directory name (`auto-installer` / `nixos-minimal`), so each one's
 generated system name (`nixos-system-<name>-*`) is self-describing rather
 than the nixpkgs default of `nixos-system-nixos-*` for both.
 
+The Debian Minimal entry chains `http://<pxeServerIp>/debian.ipxe`, which loads
+the Debian bookworm netboot kernel and initrd from `/srv/pxe/http/debian/`. The
+`fetch-debian-netboot.service` oneshot downloads these files from
+`deb.debian.org` on first boot (idempotent — skips if files are already
+present):
+
+```text
+/srv/pxe/http/debian/linux      (Debian bookworm netboot kernel)
+/srv/pxe/http/debian/initrd.gz  (Debian bookworm netboot initrd)
+```
+
+The service requires outbound internet access on the pxe-boot host. To
+re-download (e.g. after a Debian point release), delete the files and restart
+the service:
+
+```bash
+rm /srv/pxe/http/debian/linux /srv/pxe/http/debian/initrd.gz
+systemctl restart fetch-debian-netboot.service
+```
+
+To update to a different Debian release, change `debianRelease` in
+`modules/build-types/pxe-boot.nix` and redeploy.
+
 The SystemRescue entry expects the source ISO at:
 
 ```text
@@ -113,6 +139,9 @@ After deployment by an operator, basic service checks are:
 ```bash
 curl http://pxe-boot/boot.ipxe
 curl http://pxe-boot/menu.ipxe
+curl http://pxe-boot/debian.ipxe
+curl -I http://pxe-boot/debian/linux
+curl -I http://pxe-boot/debian/initrd.gz
 curl http://pxe-boot/systemrescue.ipxe
 curl -I http://pxe-boot/systemrescue/sysresccd/boot/x86_64/vmlinuz
 curl -I http://pxe-boot/systemrescue/sysresccd/boot/x86_64/sysresccd.img