feat(host): script-facing stream-active runtime marker file

Maintain $XDG_RUNTIME_DIR/punktfunk/stream while any client is streaming,
holding the primary session's negotiated mode. A per-title launch wrapper
can branch on it: present → session is already at the stream mode, run the
game as-is; absent → run the local (e.g. multi-head gamescope) path.

- New stream_marker module: RAII Guard registered per session, refcounted
  for concurrent clients, atomic (temp+rename) writes, injection-safe
  single-quoted client name. POSIX-sh-sourceable KEY=value, namespaced
  PF_STREAM_* keys, schema-versioned, additive-only.
- Hooked into serve_session so every exit path (disconnect, error,
  panic-unwind) retracts the marker. File exists iff a stream is live.

Unblocks the downstream triple-head gamescope launch-wrapper use case.

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

crates/punktfunk-host/src/main.rs

@@ -71,6 +71,7 @@ mod session_plan;
 mod session_tuning;
 mod spike;
 mod stats_recorder;
+mod stream_marker;
 mod vdisplay;
 #[cfg(target_os = "windows")]
 #[path = "windows/win_adapter.rs"]