Add SteamOS distro

Adds `--distro steamos` as a first-class build target. Pattern is the
same as bazzite-deck/batocera: distros/steamos/build-rootfs.sh fetches
the upstream rootfs (Valve's Steam Deck recovery .img.bz2 from
steamdeck-images.steamos.cloud), extracts the rootfs-A + var-A btrfs
slots into $CHROOT, swaps in our linux-ps5 kernel pkg.tar.zst, and
rebuilds the initramfs with the modules amdgpu needs for the PS5
Oberon GPU.

PS5-specific fixes applied to the SteamOS rootfs:

- Replace /usr/bin/steamos-session-select with a shim that writes
  /etc/sddm.conf.d/zzz-session-override.conf directly + restarts sddm.
  The vendored helper goes through steamosctl / steamos-manager whose
  Deck-hardware code paths silently no-op on PS5, leaving the UI stuck
  at "Switching to Desktop..." with no actual session swap.

- Install a small Python dbus monitor (steam-session-switch-listener)
  that catches org.freedesktop.DisplayManager.Seat.SwitchToUser from
  Big Picture's "Switch to Desktop" button and writes the same
  override file. No -steamos3 / -steampal needed (those trigger Steam's
  A/B update reboot loop on a non-Deck box).

- Drop a Return-to-Gaming-Mode.desktop into /home/deck/Desktop pointing
  at steamos-session-select gamescope.

- chown /home/deck to deck:deck (the recovery image leaves it root:root,
  which breaks ~/.config writability on first Plasma launch).

- Enable sshd + set deck:deck so the box is reachable for further
  iteration without dragging a USB keyboard around.

- Patch /etc/fstab: comment out the by-partsets atomic mount entries
  (those paths don't exist on a flat-rootfs flash), append a LABEL-based
  /efi mount so the FAT boot partition comes up cleanly. Required for
  ps5-iw620-firmware.service to find the WLAN blob.

- Append amdgpu.dpm=0 + amdgpu.gpu_recovery=0 to the kernel cmdline.
  modprobe.d alone isn't enough — amdgpu loads before the rootfs conf
  is read, so the cmdline is the only reliable path. Without these
  Switch-to-Desktop produces a black screen on the PS5 HDMI output.

- Polkit-deny the deck user reboot/poweroff to break Steam's update
  reboot loop on first boot.

- Patch steam-jupiter to skip its OOBE rm -rf path. The OOBE assumes
  Deck factory layout and wipes /home/deck/.local if it runs.

Verified end-to-end on real PS5 (HW-rev EA-1102): autologin lands in
gamescope, Switch-to-Desktop and Return-to-Gaming both work, sshd is
reachable, ps5-iw620 brings up internal WiFi after firmware staging.

Files:
- distros/steamos/build-rootfs.sh  (646 lines)
- distros/steamos/README.md
- distros/steamos/return-to-gaming-mode.desktop
- distros/steamos/steam-session-switch-listener.{py,service}
- build_image.sh: steamos → FORMAT=arch (uses linux-ps5 pkg.tar.zst)
- .github/workflows/build-image.yml: steamos in matrix options + distros list

.github/workflows/build-image.yml

@@ -8,7 +8,7 @@ on:
         description: 'Distribution'
         default: 'ubuntu2604'
         type: choice
-        options: [all, ubuntu2604, arch, cachyos, fedora, proxmox, debian, bazzite, bazzite-deck]
+        options: [all, ubuntu2604, arch, cachyos, fedora, proxmox, debian, bazzite, bazzite-deck, steamos]
   workflow_call:
     inputs:
       distro:
@@ -34,7 +34,7 @@ jobs:
         run: |
           INPUT="${{ inputs.distro || 'ubuntu2604' }}"
           if [ "$INPUT" = "all" ]; then
-            echo 'distros=["ubuntu2604","arch","cachyos","fedora","proxmox","debian","bazzite","bazzite-deck"]' >> "$GITHUB_OUTPUT"
+            echo 'distros=["ubuntu2604","arch","cachyos","fedora","proxmox","debian","bazzite","bazzite-deck","steamos"]' >> "$GITHUB_OUTPUT"
           else
             echo "distros=[\"$INPUT\"]" >> "$GITHUB_OUTPUT"
           fi