feat(nixos): replace X1 with ThinkPad T400; add Mac Pro 3,1 desktop

- lyrathorpe-t400 replaces lyrathorpe-x1c: ThinkPad T400 (legacy BIOS -> GRUB,
  Intel microcode + redistributable firmware for iwlwifi, pipewire, sshd).
- lyrathorpe-macpro31: new desktop host (portable = false) importing
  desktop.nix. Mac Pro 3,1 has 64-bit EFI -> systemd-boot; wired NetworkManager
  via desktop.nix; desktop status bar (temperature + net, no battery).

Both ship hand-written placeholder hardware-configuration.nix (root/swap/ESP by
label, GRUB device /dev/sda) to be regenerated with nixos-generate-config and
committed at install time. All five host configs evaluate; nixfmt clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/MacPro31/hardware-configuration.nix

@@ -0,0 +1,52 @@
+# PLACEHOLDER -- hand-written, not machine-generated. Regenerate on the real
+# Mac Pro 3,1 with `nixos-generate-config` and commit the result. The device
+# labels below are guesses; replace them with the generated UUIDs (or label the
+# partitions accordingly at install time).
+{
+  config,
+  lib,
+  modulesPath,
+  ...
+}:
+
+{
+  imports = [
+    (modulesPath + "/installer/scan/not-detected.nix")
+  ];
+
+  boot.initrd.availableKernelModules = [
+    "ahci"
+    "ata_piix"
+    "ehci_pci"
+    "uhci_hcd"
+    "firewire_ohci"
+    "usb_storage"
+    "sd_mod"
+    "sr_mod"
+  ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ "kvm-intel" ];
+  boot.extraModulePackages = [ ];
+
+  # EFI System Partition for systemd-boot. Label the ESP `ESP` at install, or
+  # replace with the generated UUID.
+  fileSystems."/boot" = {
+    device = "/dev/disk/by-label/ESP";
+    fsType = "vfat";
+  };
+
+  # Label your root partition `nixos` at install, or replace with the generated UUID.
+  fileSystems."/" = {
+    device = "/dev/disk/by-label/nixos";
+    fsType = "ext4";
+  };
+
+  swapDevices = [
+    { device = "/dev/disk/by-label/swap"; }
+  ];
+
+  networking.useDHCP = lib.mkDefault true;
+
+  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
+  hardware.cpu.intel.updateMicrocode = lib.mkDefault config.hardware.enableRedistributableFirmware;
+}