feat(host): hold a logind sleep/idle inhibitor while clients stream

Remote INPUT resets the compositor's idle timers, but a passive
(video-only) viewer sends none — observed live: a SteamOS Game-Mode
host s2idled mid-day and dropped off the network (in a VM with GPU
passthrough it never woke again). Refcounted across planes: the native
LiveSessionGuard and the GameStream video worker each hold a share;
first hold takes the logind sleep:idle block inhibitor on a dedicated
plain thread (zbus blocking must not run on a tokio worker), last drop
releases. Best-effort: no logind → warn once and stream on. No-op off
Linux (macOS/Windows manage their own assertions).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/session_status.rs

@@ -114,12 +114,18 @@ pub fn register(
         session: session_ref(&session),
     });
     registry().lock().unwrap().push(session);
-    LiveSessionGuard { id }
+    LiveSessionGuard {
+        id,
+        _sleep: crate::sleep_inhibit::hold(),
+    }
 }
 
 /// Removes its session from the registry when dropped (any scope exit of the native video loop).
 pub struct LiveSessionGuard {
     id: u64,
+    /// While any native session lives, the box must not auto-suspend under a passive viewer
+    /// ([`crate::sleep_inhibit`]) — refcounted, released with the guard.
+    _sleep: crate::sleep_inhibit::StreamHold,
 }
 
 impl Drop for LiveSessionGuard {