fix(host): input follows session per-connect + restore-guard on desktop switch

Two fixes from live Bazzite testing of the managed-Gaming + mid-stream work:

1. Input now FOLLOWS the active session. The host-lifetime injector was pinned to
   the first backend it opened and only reopened on an inject FAILURE — but with
   Feature A keeping the managed gamescope warm, its EIS socket stays alive, so a
   switch to the KDE desktop + reconnect kept injecting into the idle gamescope
   (input silently dead on KDE). injector_service_thread now compares the
   resolved input backend (default_backend() ← PUNKTFUNK_INPUT_BACKEND, set per
   connect by apply_input_env, and on a mid-stream switch) each event and reopens
   when it changes. Fixes input on a Gaming->Desktop reconnect AND Feature B's
   mid-stream input re-route, with no plumbing.

2. Debounced TV-restore no longer yanks you back to gaming. do_restore_tv_session
   now checks detect_active_session(): if a desktop session is active (the user
   switched), it tears down the idle managed gamescope but does NOT restart the
   gaming autologin. Observed live: the restore fired and restarted
   gamescope-session-plus@ogui-steam while the client was already on the KDE
   desktop.

Also: document PUNKTFUNK_SESSION_WATCH (Feature B opt-in) in the Bazzite host.env
and correct the managed-default description. Compiles, clippy/fmt clean, 78 tests.

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

crates/punktfunk-host/src/m3.rs

@@ -988,21 +988,36 @@ const INJECTOR_REOPEN_BACKOFF: std::time::Duration = std::time::Duration::from_s
 /// sender have dropped (host shutdown), which drops the injector and closes its portal session.
 fn injector_service_thread(rx: std::sync::mpsc::Receiver<InputEvent>) {
     let mut injector: Option<Box<dyn crate::inject::InputInjector>> = None;
+    let mut open_backend: Option<crate::inject::Backend> = None;
     let mut last_failed: Option<std::time::Instant> = None;
     for ev in rx {
+        // The resolved input backend (PUNKTFUNK_INPUT_BACKEND, set per connect by apply_input_env,
+        // also on a mid-stream session switch) may have changed since we opened. Reopen against it
+        // so input FOLLOWS the active session instead of injecting into a stale, still-warm backend
+        // (e.g. the managed gamescope's EIS socket after the user switched to the KDE desktop).
+        let want = crate::inject::default_backend();
+        if injector.is_some() && open_backend != Some(want) {
+            tracing::info!(
+                ?open_backend,
+                ?want,
+                "input: backend changed — reopening injector for the active session"
+            );
+            injector = None;
+            last_failed = None; // re-resolve immediately
+        }
         if injector.is_none() {
             // Open on the first event; after a failure wait out the backoff before retrying (a
             // few events drop during setup — acceptable, input is lossy).
             let ready = last_failed.is_none_or(|t| t.elapsed() >= INJECTOR_REOPEN_BACKOFF);
             if ready {
-                let backend = crate::inject::default_backend();
-                match crate::inject::open(backend) {
+                match crate::inject::open(want) {
                     Ok(i) => {
                         tracing::info!(
-                            ?backend,
+                            backend = ?want,
                             "punktfunk/1 input injector ready (host-lifetime)"
                         );
                         injector = Some(i);
+                        open_backend = Some(want);
                         last_failed = None;
                     }
                     Err(e) => {
@@ -1018,6 +1033,7 @@ fn injector_service_thread(rx: std::sync::mpsc::Receiver<InputEvent>) {
                 // a later event (covers a gamescope EIS socket that respawns with its session).
                 tracing::warn!(error = %format!("{e:#}"), "inject failed — reopening injector");
                 injector = None;
+                open_backend = None;
                 last_failed = Some(std::time::Instant::now());
             }
         }

crates/punktfunk-host/src/vdisplay/gamescope.rs

@@ -266,6 +266,19 @@ fn do_restore_tv_session() {
     }
     stop_session(SESSION_UNIT); // our gamescope/Steam session, so Steam is free for the autologin
     *MANAGED_SESSION.lock().unwrap_or_else(|e| e.into_inner()) = None;
+    // Only bring the gaming autologin BACK if the box is still meant to be in gaming mode. If the
+    // user switched to a desktop session (KDE/GNOME/wlroots) in the meantime, don't yank them back
+    // to gaming — leave the desktop alone. (We still stopped our idle managed session above.)
+    use super::ActiveKind;
+    if matches!(
+        super::detect_active_session().kind,
+        ActiveKind::DesktopKde | ActiveKind::DesktopGnome | ActiveKind::DesktopWlroots
+    ) {
+        tracing::info!(
+            "gamescope: a desktop session is active — not restoring the TV gaming session"
+        );
+        return;
+    }
     for unit in units {
         let _ = Command::new("systemctl")
             .args(["--user", "start", &unit])

packaging/bazzite/host.env

@@ -1,8 +1,8 @@
 # punktfunk host config for Bazzite (~/.config/punktfunk/host.env).
 #
 # The compositor + input backend are AUTO-DETECTED per connect from the ACTIVE session: the host
-# follows the box as you flip between Steam Gaming Mode (gamescope — attached to the running
-# session, no churn) and a KDE/GNOME Desktop (KWin/Mutter virtual output at the client's mode).
+# follows the box as you flip between Steam Gaming Mode (gamescope — a managed session at the
+# CLIENT's resolution) and a KDE/GNOME Desktop (KWin/Mutter virtual output at the client's mode).
 # So nothing here forces a backend — only the trustworthy anchors stay.
 
 XDG_RUNTIME_DIR=/run/user/1000
@@ -26,3 +26,6 @@ PUNKTFUNK_ZEROCOPY=1
 # stays live on the panel, no Steam restart), set:
 #   PUNKTFUNK_GAMESCOPE_ATTACH=1
 #   PUNKTFUNK_GAMESCOPE_APP=steam -gamepadui   # only for an ad-hoc bare-spawn fallback
+#
+# Follow a Gaming<->Desktop switch MID-STREAM (rebuild the backend in place, no reconnect):
+#   PUNKTFUNK_SESSION_WATCH=1