fix(vdisplay/session): scrub the dead desktop's WAYLAND_DISPLAY from the user-manager env

settle_desktop_portal pushes the live desktop's WAYLAND_DISPLAY into the
systemd --user manager (import-environment) so a re-activated portal inherits
the right session — but the import PERSISTS after that desktop dies. Every
later user unit then inherits the stale socket, including
gamescope-session.target: gamescope sees WAYLAND_DISPLAY, runs NESTED, and
aborts with "Failed to connect to wayland socket: wayland-0" — observed live
on a Deck, where Game Mode could not start at all (autologin crash-looped)
until the var was unset by hand.

Two-layer fix, mirroring the protection launch_session already gives its
transient unit:
- observe_session_instance: when a desktop compositor instance goes away,
  unset WAYLAND_DISPLAY/DISPLAY from the manager env (a desktop bounce is
  harmless — the next portal settle re-imports).
- write_steamos_dropin: UnsetEnvironment=DISPLAY WAYLAND_DISPLAY on the
  takeover drop-in, unit-scoped belt-and-suspenders for host-initiated
  restarts.

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

crates/pf-vdisplay/src/vdisplay/linux/gamescope.rs

@@ -630,12 +630,17 @@ fn write_steamos_dropin(shim_dir: &std::path::Path, mode: Mode) -> Result<()> {
     if let Some(parent) = path.parent() {
         std::fs::create_dir_all(parent).with_context(|| format!("mkdir {}", parent.display()))?;
     }
+    // UnsetEnvironment: the same headless-must-not-attach armor `launch_session` gives its
+    // transient unit — the manager env can carry a stale desktop DISPLAY/WAYLAND_DISPLAY (from a
+    // portal settle), and gamescope would abort trying to attach to it instead of becoming the
+    // display server. Unit-scoped belt-and-suspenders on top of the observe_session_instance scrub.
     let body = format!(
         "[Service]\n\
          Environment=PATH={shim}:/usr/bin:/bin:/usr/local/bin\n\
          Environment=PF_W={w}\n\
          Environment=PF_H={h}\n\
-         Environment=PF_HZ={hz}\n",
+         Environment=PF_HZ={hz}\n\
+         UnsetEnvironment=DISPLAY WAYLAND_DISPLAY\n",
         shim = shim_dir.display(),
         w = mode.width,
         h = mode.height,