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/punktfunk1.rs

@@ -1582,6 +1582,17 @@ async fn serve_session(
     let source = opts.source;
     let (seconds, frames) = (opts.seconds, opts.frames);
     let mode = hello.mode;
+    // Script-facing runtime marker: `$XDG_RUNTIME_DIR/punktfunk/stream` exists (with this session's
+    // negotiated mode) for exactly as long as this session streams. Held by RAII to session end, so
+    // every exit path — clean disconnect, error, panic-unwind — retracts it. Lets a launch wrapper
+    // branch "streaming → run the game as-is; not → my local multi-head gamescope" (see the module).
+    let _stream_marker = crate::stream_marker::announce(crate::stream_marker::StreamInfo {
+        width: mode.width,
+        height: mode.height,
+        refresh_hz: mode.refresh_hz,
+        hdr: welcome.color.is_hdr(),
+        client: hello.name.clone().unwrap_or_default(),
+    });
     // The session's launch, threaded into the data plane. Windows carries the store-qualified id
     // (spawned into the interactive user session once capture is live); other hosts resolve the id
     // to its shell command HERE against the host's own library — a client can only ever pick an