bump to nix 26.05

nix/hosts/placeholder-hardware.nix

@@ -0,0 +1,24 @@
+{ lib, ... }:
+
+# SKELETON PLACEHOLDER — not a real machine description.
+#
+# The three hosts don't have actual boxes yet, but `nix flake check` evaluates
+# `system.build.toplevel` for every nixosConfiguration, which asserts a root
+# filesystem and a boot device exist. This stub satisfies those assertions so
+# the structure type-checks in CI.
+#
+# When a real machine is provisioned, generate its hardware-configuration.nix
+# (`nixos-generate-config`) and import that from the host file instead of this
+# placeholder (see the commented import in db-host.nix). Do NOT deploy a host
+# that still relies on this file — the device paths below are fictional.
+{
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/nixos";
+    fsType = "ext4";
+  };
+
+  boot.loader.grub = {
+    enable = true;
+    devices = [ "/dev/sda" ];
+  };
+}