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/gamestream/stream.rs

@@ -60,6 +60,9 @@ pub fn start(
             // Same scheduling posture as the native path's capture/encode thread (Linux nice -10 /
             // Windows HIGHEST + session tuning) — GameStream previously ran unboosted on Linux.
             crate::native::boost_thread_priority(true);
+            // A GameStream viewer may be video-only too — hold the suspend/idle inhibitor for
+            // this stream's lifetime (plane parity with the native LiveSessionGuard).
+            let _sleep = crate::sleep_inhibit::hold();
             tracing::info!(?cfg, "video stream starting");
             // Lifecycle events + the script-facing marker file, plane parity with the native loop
             // (RFC §4): `announce` emits `stream.started`/`stream.stopped` and holds the marker for