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.

flake.nix

@@ -65,7 +65,7 @@
           # file without a same-option circular dependency (a module
           # contributing to environment.etc can't read the merged
           # environment.etc it's itself contributing to).
-          specialArgs = { inherit inputs vars netbootSystem flakeTarget; };
+          specialArgs = { inherit inputs vars netbootSystem netbootMinimalSystem flakeTarget; };
         };
 
       # Generated platform x build-type matrix. pxe-boot has no linode
@@ -133,6 +133,12 @@
       # profiles/installation-device.nix independently, so common.nix's
       # initialHashedPassword override (which assumes that profile is
       # present) still applies correctly without iso.nix in the mix.
+      #
+      # networking.hostName is set explicitly (rather than left at nixpkgs'
+      # own "nixos" default) so this image's generated system name
+      # (nixos-system-auto-installer-*) matches its iPXE menu entry —
+      # see modules/build-types/pxe-boot.nix's :auto-installer item — and
+      # its staged directory, /srv/pxe/http/auto-installer.
       netbootSystem = nixpkgs.lib.nixosSystem {
         inherit system;
         modules = [
@@ -142,10 +148,32 @@
               (modulesPath + "/installer/netboot/netboot-minimal.nix")
             ];
           })
+          { networking.hostName = "auto-installer"; }
         ];
         specialArgs = { inherit vars; };
       };
 
+      # A genuinely vanilla NixOS minimal netboot image: nixpkgs'
+      # netboot-minimal.nix on its own, with none of this flake's
+      # auto-installer wiring (no common.nix — no auto-install.sh, no
+      # baked host keys, no custom users/passwords). Built from source via
+      # the same nixosSystem + netboot-minimal.nix path as netbootSystem
+      # above, so both go through an identical build mechanism; the only
+      # difference is what's composed in. hostName again matches this
+      # image's iPXE menu entry (:nixos-minimal) and staged directory
+      # (/srv/pxe/http/nixos-minimal).
+      netbootMinimalSystem = nixpkgs.lib.nixosSystem {
+        inherit system;
+        modules = [
+          ({ modulesPath, ... }: {
+            imports = [
+              (modulesPath + "/installer/netboot/netboot-minimal.nix")
+            ];
+          })
+          { networking.hostName = "nixos-minimal"; }
+        ];
+      };
+
     in
     {
 
@@ -167,6 +195,15 @@
           { name = "initrd"; path = netbootSystem.config.system.build.netbootRamdisk; }
           { name = "kernel"; path = netbootSystem.config.system.build.kernel; }
         ];
+
+        # Vanilla NixOS minimal netboot bundle — see netbootMinimalSystem
+        # above. Staged onto the pxe-boot host alongside packages.pxe by
+        # modules/pxe-boot/stage-installer-artifacts.nix.
+        pxe-minimal = pkgs.linkFarm "pxe-minimal" [
+          { name = "netboot.ipxe"; path = netbootMinimalSystem.config.system.build.netbootIpxeScript; }
+          { name = "initrd"; path = netbootMinimalSystem.config.system.build.netbootRamdisk; }
+          { name = "kernel"; path = netbootMinimalSystem.config.system.build.kernel; }
+        ];
       };
     };
 }