Rename PXE installer menu entry, add vanilla NixOS minimal netboot entry

The iPXE menu's "nixos" entry actually chain-loads this flake's own
custom auto-installer image, not a stock NixOS image — rename it to
"auto-installer" (label "NixOS Auto-Installer") so the menu says what it
boots, and set networking.hostName on netbootSystem to match, so the
generated system name (nixos-system-auto-installer-*) and staged
directory (/srv/pxe/http/auto-installer) agree with the menu entry too.

Add a second, genuinely vanilla NixOS minimal netboot image
(netbootMinimalSystem in flake.nix — nixpkgs' netboot-minimal.nix on its
own, none of modules/installer/common.nix's auto-installer wiring),
built from source the same way as the auto-installer image and exposed
as packages.x86_64-linux.pxe-minimal. Staged and menu-wired the same
way, as "nixos-minimal" (item, hostname, and directory all matching).

modules/pxe-boot/stage-installer-artifacts.nix is generalized to stage
both images via a shared rule-builder instead of one hardcoded set of
paths.

Verified: nix eval confirms both images' config.system.name matches
their menu entry/directory names, the pxe-boot host itself builds
clean with the new menu.ipxe, and the new pxe-minimal image was booted
directly under QEMU (kernel+initrd, no KVM) to a working login shell
with hostname nixos-minimal, no hang.

modules/pxe-boot/stage-installer-artifacts.nix

@@ -1,23 +1,32 @@
-{ netbootSystem, ... }:
+{ netbootSystem, netbootMinimalSystem, ... }:
 
 let
   # config.system.build.kernel and .netbootRamdisk are directories, not the
   # files themselves — nixpkgs' own system.build.kexecTree does the same
   # ${...}/<file> dereference for the same reason.
-  inherit (netbootSystem.config.system.boot.loader) kernelFile;
+  mkStageRules = { dirName, system }:
+    let
+      inherit (system.config.system.boot.loader) kernelFile;
+      dir = "/srv/pxe/http/${dirName}";
+    in
+    [
+      # Declared here too (not just in build-types/pxe-boot.nix) so this
+      # module's C+ rules don't depend on cross-module list-merge ordering —
+      # tmpfiles' C type needs the target directory to already exist.
+      "d ${dir} 0755 root root -"
+      "C+ ${dir}/${kernelFile} 0644 root root - ${system.config.system.build.kernel}/${kernelFile}"
+      "C+ ${dir}/initrd 0644 root root - ${system.config.system.build.netbootRamdisk}/initrd"
+      "C+ ${dir}/netboot.ipxe 0644 root root - ${system.config.system.build.netbootIpxeScript}/netboot.ipxe"
+    ];
 in
 {
   # Builds this flake's own installer netboot image (the same one
-  # `nix build .#pxe` produces) and stages it where menu.ipxe's :nixos
-  # entry expects it, so the pxe-boot host is self-contained — no manual
-  # operator step to populate /srv/pxe/http/nixos after deploy.
-  systemd.tmpfiles.rules = [
-    # Declared here too (not just in build-types/pxe-boot.nix) so this
-    # module's C+ rules don't depend on cross-module list-merge ordering —
-    # tmpfiles' C type needs the target directory to already exist.
-    "d /srv/pxe/http/nixos 0755 root root -"
-    "C+ /srv/pxe/http/nixos/${kernelFile} 0644 root root - ${netbootSystem.config.system.build.kernel}/${kernelFile}"
-    "C+ /srv/pxe/http/nixos/initrd 0644 root root - ${netbootSystem.config.system.build.netbootRamdisk}/initrd"
-    "C+ /srv/pxe/http/nixos/netboot.ipxe 0644 root root - ${netbootSystem.config.system.build.netbootIpxeScript}/netboot.ipxe"
-  ];
+  # `nix build .#pxe` produces) plus the vanilla NixOS minimal netboot image
+  # (`nix build .#pxe-minimal`), and stages both where menu.ipxe's
+  # :auto-installer / :nixos-minimal entries expect them, so the pxe-boot
+  # host is self-contained — no manual operator step to populate
+  # /srv/pxe/http after deploy.
+  systemd.tmpfiles.rules =
+    mkStageRules { dirName = "auto-installer"; system = netbootSystem; }
+    ++ mkStageRules { dirName = "nixos-minimal"; system = netbootMinimalSystem; };
 }