feat(macpro31): NVIDIA Quadro P400 driver and CUDA-enabled Docker

The stock GPU has been replaced with a Quadro P400 (Pascal, GP108). Add
hosts/MacPro31/nvidia.nix:

- Driver branch 580 (nvidiaPackages.legacy_580), not the nixpkgs default
  production branch (595.x). 580 is the last branch supporting
  Maxwell/Pascal/Volta and is an LTS branch until Aug 2028; a newer one
  does not drive this card.
- modesetting.enable for Wayland (nvidia-drm.modeset=1), open = false
  (the open kernel modules need Turing or later), and sway
  --unsupported-gpu, which wlroots requires with the proprietary driver.
- Docker with GPU access via CDI (hardware.nvidia-container-toolkit),
  rather than the deprecated virtualisation.docker.enableNvidia runtime
  wrapper. Containers run with --device=nvidia.com/gpu=all and must ship
  a CUDA 12.x or older runtime: CUDA 13 dropped sm_61.

The driver packages are unfree, so allowlist them in unfreePackages; they
are not cached and the kernel module builds on the host.

Also declare features.cpu.microarchLevel = 1 for this machine: the
Harpertown Xeons have SSE4.1 but no SSE4.2/POPCNT, which switches off
Claude Code through the fleet-wide gate.

hosts/MacPro31/configuration.nix

@@ -7,8 +7,14 @@
 {
   imports = [
     ./hardware-configuration.nix
+    ./nvidia.nix
   ];
 
+  # Dual quad-core Xeon (Harpertown/Penryn): SSE4.1 but no SSE4.2 or POPCNT,
+  # i.e. x86-64-v1. Declaring it here switches off the fleet flags that need a
+  # newer CPU -- currently features.claudeCode (see ../../modules/features.nix).
+  features.cpu.microarchLevel = 1;
+
   # The Mac Pro 3,1 has 64-bit EFI (confirmed by the owner), so boot via
   # systemd-boot like the MBP -- no GRUB/BIOS shim needed.
   boot.loader.systemd-boot.enable = true;
@@ -33,17 +39,9 @@
   # enabled in workstation.nix.
   hardware.cpu.intel.updateMicrocode = true;
 
-  # GPU note: the stock card varies between units -- ATI Radeon HD 2600 XT or
-  # NVIDIA GeForce 8800 GT. Sway needs a working KMS/modesetting driver; do NOT
-  # install a proprietary blob here. Depending on the installed card, rely on
-  # the open kernel driver:
-  #   - ATI Radeon HD 2600 XT -> "radeon" (older) or "amdgpu" KMS
-  #   - NVIDIA GeForce 8800 GT -> "nouveau" KMS
-  # These come up automatically via the in-tree drivers + KMS, and the graphics
-  # stack itself is enabled by modules/sway.nix. If a card needs to be forced, add it
-  # here, e.g. `services.xserver.videoDrivers = [ "radeon" ];` (or "nouveau"),
-  # and/or `boot.initrd.kernelModules = [ "radeon" ];` in
-  # hardware-configuration.nix for early KMS.
+  # GPU: the stock card (ATI Radeon HD 2600 XT / NVIDIA GeForce 8800 GT) has
+  # been replaced with an NVIDIA Quadro P400. Driver, Wayland quirks and
+  # GPU-enabled Docker live in ./nvidia.nix.
 
   # See `man configuration.nix` / the stateVersion docs before changing.
   system.stateVersion = "26.05";