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/rpm/punktfunk.spec

@@ -233,6 +233,13 @@ install -Dm0644 packaging/kde/host.env                 %{buildroot}%{_datadir}/%
 # screencast/virtual-output grant ships as io.unom.Punktfunk.Host.desktop, installed above).
 install -d %{buildroot}%{_datadir}/%{name}/bazzite
 install -Dm0755 packaging/bazzite/kde-desktop-setup.sh %{buildroot}%{_datadir}/%{name}/bazzite/kde-desktop-setup.sh
+# Headless GAME-mode fix: a gamescope-session-plus sessions.d drop-in that falls back to gamescope's
+# headless backend when no display is connected (so "Switch to Game Mode" works on a display-less
+# streaming host instead of crashing + 5-striking back to desktop). No-op on display-attached boxes.
+# Sourced by gamescope-session-plus as /etc/gamescope-session-plus/sessions.d/steam (after its
+# /usr/share defaults). Harmless on non-gamescope systems (the file is simply never read).
+install -Dm0644 packaging/bazzite/gamescope-headless-session \
+                %{buildroot}/etc/gamescope-session-plus/sessions.d/steam
 install -Dm0644 api/openapi.json                  %{buildroot}%{_datadir}/%{name}/openapi.json
 
 %if %{with web}
@@ -262,6 +269,9 @@ install -Dm0644 web/web.env.example                %{buildroot}%{_datadir}/punkt
 %{_userunitdir}/punktfunk-host.service
 %{_userunitdir}/punktfunk-kde-session.service
 %{_datadir}/applications/io.unom.Punktfunk.Host.desktop
+%dir /etc/gamescope-session-plus
+%dir /etc/gamescope-session-plus/sessions.d
+%config(noreplace) /etc/gamescope-session-plus/sessions.d/steam
 %dir %{_datadir}/%{name}
 %{_datadir}/%{name}/*