feat(hardware): xe-aware GuC, class-based NPU detect, latestKernel + VM test

Harden the hardware-enablement broad seed for newer hardware:

- xe vs i915: intel.guc emits the i915 param (i915.enable_guc=3), which the
  newer `xe` driver (Lunar Lake / Battlemage / Panther Lake, recent Xe GPUs)
  ignores — it enables GuC by default. hardware-db.sh now reads the in-use
  GPU driver (lspci -k) and writes `intel.guc = false` on xe hardware, so the
  toggle isn't a misleading no-op there.

- NPU detection by PCI accelerator class [1200] (+ keywords), attributing the
  vendor, instead of a fixed device-ID list — so new NPUs (e.g. Panther Lake)
  are caught without a code change. Known IDs kept as a fallback/reference.

- Kernel awareness: a nomarchy.hardware.latestKernel escape hatch
  (linuxPackages_latest) for very-new hardware whose drivers only just landed,
  and a build-time warning when npu.enable predates the shipped kernel
  (amdxdna needs 6.14+, intel_vpu wants recent). Downstream default is 6.18,
  which already carries amd-pstate + amdxdna; the reference host pins latest.

- VM test (checks.hardware-toggles, pkgs.testers.runNixOSTest): boots a
  minimal VM with the toggles on and asserts the config landed — amd_pstate
  + i915.enable_guc in /proc/cmdline, fprintd.service present, pam_fprintd in
  /etc/pam.d/sudo. Passing. (Hardware behaviour still needs bare metal.)

Docs: README + template note latestKernel; ROADMAP records the hardening.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/downstream/system.nix

@@ -40,6 +40,7 @@
   #   fingerprint.enable = true;                #   fprintd; installer-set when a reader is detected
   #   fingerprint.pam = true;                   #   use the fingerprint for login + sudo
   #   npu.enable = true;                        #   on-die NPU driver (experimental; userspace runtime BYO)
+  #   latestKernel = true;                       #   newest kernel for very-new hardware (drivers not yet in the default)
   # };
   #
   # nomarchy.services.tailscale.enable = true; # mesh VPN — then `sudo tailscale up`