64799e55

By: Ammar <ammar.nanjiani@gmail.com>

feat(steammachine): add Jovian NixOS host for Valve fremont hardware

Replace stock SteamOS on the Steam Machine's internal NVMe with Jovian
NixOS. Jovian's Steam Machine support is unmerged upstream (PR #584), so
this pins the author's fork branch as a separate input. Only this host
imports it; steamdeck stays on chaotic's vendored Jovian.

Notable choices:

- GRUB with efiInstallAsRemovable. efibootmgr on the machine shows no
  explicit OS boot entry, only an {auto_created_boot_option} for the
  NVMe, so the firmware resolves \EFI\BOOT\BOOTX64.EFI. An NVRAM-only
  install would leave the box unable to boot.
- The Jovian overlay is applied by the host config. Jovian's
  modules/default.nix does not apply it, and the Steam Machine modules
  reference pkgs.linuxPackages_jovian, pkgs.fremont-hw-support and
  pkgs.mesa-{radeonsi,radv}-jupiter.
- BIOS updates stay manual. enableFwupdBiosUpdates is on, but autoUpdate
  keeps its default of false: a BIOS flash cannot be rolled back by a
  NixOS generation.
- Theoden's game archive is mounted read-only with a host-local
  systemd.mounts block rather than modules/nixos/nfs-client.nix, which
  hardcodes its mount flags and exposes no read-only option.
- home-manager is managed by the NixOS module, so one nixos-anywhere run
  covers both system and user config.

hardware-configuration.nix is still a placeholder. nixos-anywhere fills
it in via --generate-hardware-config during install.