Implement Cast Streaming mirroring, DLNA casting, daemon+GUI, and breadd integration

Builds out the full v1 scope: a vendored+patched openscreen subset for
low-latency Cast Streaming (Mirroring receiver 0F5096E8) alongside the
existing Cast V2/HLS and new DLNA/AVTransport casting paths, breadcastd's
Idle/Casting state machine with a private IPC socket, the breadcast GTK4
popup as a thin IPC client, and bread.cast.*/bread.command.cast.* breadd
integration (device discovery, start/stop, mirroring lifecycle events).
Also adds bakery/systemd/Forgejo CI packaging.

Validated end-to-end against a real Chromecast/Google TV: negotiated
Cast Streaming session, live pipeline playback, and daemon+GUI click-to-cast/
stop through the actual popup.

breadcast-caststream-sys/vendor/openscreen/util/thread_annotations.h

@@ -0,0 +1,50 @@
+// Copyright 2026 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UTIL_THREAD_ANNOTATIONS_H_
+#define UTIL_THREAD_ANNOTATIONS_H_
+
+#if defined(__clang__) && !defined(SWIG)
+#define OSP_THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))
+#else
+#define OSP_THREAD_ANNOTATION_ATTRIBUTE__(x)
+#endif
+
+#define OSP_GUARDED_BY(x) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(guarded_by(x))
+
+#define OSP_PT_GUARDED_BY(x) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(pt_guarded_by(x))
+
+#define OSP_EXCLUSIVE_LOCKS_REQUIRED(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(__VA_ARGS__))
+
+#define OSP_SHARED_LOCKS_REQUIRED(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(shared_locks_required(__VA_ARGS__))
+
+#define OSP_EXCLUSIVE_LOCK_FUNCTION(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_lock_function(__VA_ARGS__))
+
+#define OSP_SHARED_LOCK_FUNCTION(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(shared_lock_function(__VA_ARGS__))
+
+#define OSP_UNLOCK_FUNCTION(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(unlock_function(__VA_ARGS__))
+
+#define OSP_LOCKS_EXCLUDED(...) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(locks_excluded(__VA_ARGS__))
+
+#define OSP_LOCK_RETURNED(x) \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(lock_returned(x))
+
+#define OSP_LOCKABLE \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(lockable)
+
+#define OSP_SCOPED_LOCKABLE \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(scoped_lockable)
+
+#define OSP_NO_THREAD_SAFETY_ANALYSIS \
+  OSP_THREAD_ANNOTATION_ATTRIBUTE__(no_thread_safety_analysis)
+
+#endif  // UTIL_THREAD_ANNOTATIONS_H_