nixos/facter: add core library and system detection

This adds foundational functionality for nixos-facter hardware detection:

- lib.nix: Internal helper functions for querying facter reports
  - hasCpu/hasAmdCpu/hasIntelCpu: CPU vendor detection
  - collectDrivers: Extract driver_modules from hardware entries
  - toZeroPaddedHex: Format USB device IDs (for fingerprint matching)

- system.nix: Auto-detect nixpkgs.hostPlatform from facter report
  Automatically sets the correct platform (x86_64-linux, aarch64-linux, etc.)
  based on the hardware report, reducing manual configuration.

This builds on the base infrastructure added in PR #450303 and provides
the foundation for upcoming hardware detection modules (boot, networking,
graphics, etc.).

Part of the incremental upstreaming effort from:
https://github.com/nix-community/nixos-facter-modules

nixos/modules/hardware/facter/default.nix

@@ -4,6 +4,10 @@
   ...
 }:
 {
+  imports = [
+    ./system.nix
+  ];
+
   meta.maintainers = with lib.maintainers; [ mic92 ];
 
   options.hardware.facter = with lib; {