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>

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