fix(vdisplay/gamescope): capture-loss rebuild probes must not steal the box session

Observed live on a Deck host: the user switched Game Mode → Desktop, KDE came
up in 0.8 s — and the capture-loss rebuild's FIRST detection ran inside that
gap, still said Gaming, and its gamescope re-acquire restarted
gamescope-session.target. On SteamOS that steals the seat: the user was
yanked out of the KDE session they had just chosen, the two session managers
fought (job canceled / dependency failed), no gamescope node appeared within
30 s, and the stream died at the 40 s rebuild budget.

A rebuild probe acting on possibly-stale detection must never stop, relaunch,
or take over box sessions. New pf_vdisplay::rebuild_probe_scope (RAII,
counted): while held, the gamescope managed/takeover create paths only attach
to a live node and fail fast otherwise — no stop_autologin_sessions, no
session-plus relaunch, no gamescope-session.target restart. The capture-loss
loop holds it for the first 4 s after a loss (the detection-ambiguity
window); after that, destructive rebuilds are allowed again, so a genuine
switch INTO Game Mode still gets its headless takeover. Probe failures are
instant, so the loop now paces itself at ~2 Hz instead of spinning.

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

crates/pf-vdisplay/src/lib.rs

@@ -254,6 +254,34 @@ pub fn detect() -> Result<Compositor> {
     }
 }
 
+/// Attach-only probes: while any scope is held, backend `create` paths must not stop, relaunch,
+/// or take over box sessions — they may only attach to an already-live output, and fail fast
+/// otherwise. The capture-loss rebuild holds one for its first seconds: right after a capture
+/// loss the active-session detection can be STALE (a Game→Desktop switch observed live: the
+/// probe's gamescope re-acquire restarted `gamescope-session.target` and yanked the user out of
+/// the KDE session they had just switched to). A counter, so overlapping scopes compose.
+static REBUILD_PROBES: std::sync::atomic::AtomicU32 = std::sync::atomic::AtomicU32::new(0);
+
+/// RAII scope marking pipeline builds as attach-only probes (see [`rebuild_probe_active`]).
+pub struct RebuildProbeScope(());
+
+pub fn rebuild_probe_scope() -> RebuildProbeScope {
+    REBUILD_PROBES.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+    RebuildProbeScope(())
+}
+
+impl Drop for RebuildProbeScope {
+    fn drop(&mut self) {
+        REBUILD_PROBES.fetch_sub(1, std::sync::atomic::Ordering::SeqCst);
+    }
+}
+
+/// Is any [`rebuild_probe_scope`] active? Destructive session operations (stop/relaunch/
+/// takeover-restart) must be skipped while true.
+pub fn rebuild_probe_active() -> bool {
+    REBUILD_PROBES.load(std::sync::atomic::Ordering::SeqCst) > 0
+}
+
 /// Open the virtual-display driver for `compositor`.
 pub fn open(compositor: Compositor) -> Result<Box<dyn VirtualDisplay>> {
     #[cfg(target_os = "linux")]