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/main.rs

@@ -72,6 +72,7 @@ mod send_pacing;
 mod service;
 mod session_plan;
 mod session_status;
+mod sleep_inhibit;
 mod spike;
 mod stats_recorder;
 // The plugin store: signed catalogs, tiered trust, and install/uninstall jobs that run through the