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/arch/PKGBUILD

@@ -72,6 +72,8 @@ package_punktfunk-host() {
               'xdg-desktop-portal-wlr: portal for the headless Sway session helper'
               'punktfunk-web: browser management console (device pairing + status)')
   install=punktfunk-host.install
+  # User-editable config: the headless game-mode drop-in (see below) — don't clobber local edits.
+  backup=('etc/gamescope-session-plus/sessions.d/steam')
   local R; R="$(_repo)"; local T="$srcdir/target/release"
 
   install -Dm0755 "$T/punktfunk-host"                 "$pkgdir/usr/bin/punktfunk-host"
@@ -100,6 +102,11 @@ package_punktfunk-host() {
   install -Dm0644 "$R/scripts/host.env.example"              "$pkgdir/usr/share/punktfunk/host.env.example"
   install -Dm0644 "$R/packaging/bazzite/host.env"            "$pkgdir/usr/share/punktfunk/host.env.bazzite"
   install -Dm0644 "$R/packaging/kde/host.env"                "$pkgdir/usr/share/punktfunk/host.env.kde"
+  # Headless GAME-mode fix: gamescope-session-plus drop-in that uses the headless backend when no
+  # display is connected (so SteamOS/Bazzite "Switch to Game Mode" works on a display-less streaming
+  # host). No-op on display-attached boxes; sourced as /etc/gamescope-session-plus/sessions.d/steam.
+  install -Dm0644 "$R/packaging/bazzite/gamescope-headless-session" \
+    "$pkgdir/etc/gamescope-session-plus/sessions.d/steam"
   install -Dm0644 "$R/api/openapi.json"                 "$pkgdir/usr/share/punktfunk/openapi.json"
   install -Dm0644 "$R/LICENSE-MIT"    "$pkgdir/usr/share/licenses/punktfunk-host/LICENSE-MIT"
   install -Dm0644 "$R/LICENSE-APACHE" "$pkgdir/usr/share/licenses/punktfunk-host/LICENSE-APACHE"