feat(host/gamescope): headless game mode that follows the box + matches the client

Make Steam game mode work on a display-less streaming host and stream it at the
client's resolution:

* Ship /etc/gamescope-session-plus/sessions.d/steam (packaging/bazzite/
  gamescope-headless-session, installed by the RPM + Arch PKGBUILD): fall back to
  gamescope's headless backend when no display is connected, so "Switch to Game
  Mode" boots offscreen instead of crashing on the missing panel (and 5-striking
  back to desktop). No-op on display-attached boxes; only sets unset values so
  the host's per-client mode still wins.

* Default Bazzite/SteamOS to ATTACH (PUNKTFUNK_GAMESCOPE_ATTACH=1 in host.env):
  the box owns its session (Desktop<->Game, persistent), the host follows +
  captures it and never tears it down — so switching is rock-solid and a
  disconnect leaves the box in its mode (reconnect returns there).

* Resize-on-attach (gamescope.rs): on connect, ensure the box's own game-mode
  session runs at the CLIENT's resolution — reuse it when already matching (fast
  path, no restart), else reconfigure + restart the box's own autologin
  gamescope-session-plus@<client> at the client mode (cooperative: no competing
  unit, so no autologin-respawn fight). Detect the live gamescope's -W/-H via
  argv[0] in /proc (its /proc/<pid>/exe is unreadable for that process).

Validated live on a headless bazzite-deck-nvidia box: game mode boots headless +
stable (0 strikes); the host attaches + streams video/audio/EIS input; a
5120x1440 client reuses the matching session and streams at 5120x1440.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packaging/bazzite/gamescope-headless-session

@@ -0,0 +1,22 @@
+# punktfunk: headless game-mode fallback for gamescope-session-plus.
+#
+# Installed as /etc/gamescope-session-plus/sessions.d/steam. The gamescope-session-plus launcher
+# SOURCES this (shell, with `set -a` so assignments auto-export) AFTER its /usr/share defaults, so it
+# can override the session's gamescope flags.
+#
+# Why: on a box with NO connected display (a dedicated streaming host), the stock Steam game mode runs
+# gamescope's DRM backend against a physical panel (`--prefer-output *,eDP-1`). With nothing to scan
+# out, gamescope crashes on launch; after 5 strikes Bazzite/SteamOS force-selects the desktop session
+# and "Switch to Game Mode" appears broken. Falling back to gamescope's HEADLESS backend makes game
+# mode render entirely offscreen and expose a PipeWire node, which the punktfunk host captures and
+# streams — full gamescope game mode (per-game res / FSR / HDR / VRR / frame-limit), no monitor needed.
+#
+# Safe by construction:
+#   * NO-OP when any display is connected -> the normal DRM game mode runs unchanged.
+#   * Only sets values that are still unset (`: "${VAR:=...}"`), so the punktfunk host's per-client
+#     mode (SCREEN_WIDTH/SCREEN_HEIGHT injected via systemd-run for a managed session) still wins.
+if ! grep -qx connected /sys/class/drm/*/status 2>/dev/null; then
+    : "${BACKEND:=headless}"
+    : "${SCREEN_WIDTH:=1920}"
+    : "${SCREEN_HEIGHT:=1080}"
+fi