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/cast/streaming/ssrc.h

@@ -0,0 +1,37 @@
+// Copyright 2019 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CAST_STREAMING_SSRC_H_
+#define CAST_STREAMING_SSRC_H_
+
+#include <stdint.h>
+
+namespace openscreen::cast {
+
+// A Synchronization Source is a 32-bit opaque identifier used in RTP packets
+// for identifying the source (or recipient) of a logical sequence of encoded
+// audio/video frames. In other words, an audio stream will have one sender SSRC
+// and a video stream will have a different sender SSRC.
+using Ssrc = uint32_t;
+
+// The "not set" or "null" value for the Ssrc type.
+inline constexpr Ssrc kNullSsrc = 0;
+
+// Computes a new SSRC that will be used to uniquely identify an RTP stream. The
+// `higher_priority` argument, if true, will generate an SSRC that causes the
+// system to use a higher priority when scheduling data transmission. Generally,
+// this is set to true for audio streams and false for video streams.
+Ssrc GenerateSsrc(bool higher_priority);
+
+// Returns a value indicating how to prioritize data transmission for a stream
+// with `ssrc_a` versus a stream with `ssrc_b`:
+//
+//   ret < 0: Stream `ssrc_a` has higher priority.
+//   ret == 0: Equal priority.
+//   ret > 0: Stream `ssrc_b` has higher priority.
+int ComparePriority(Ssrc ssrc_a, Ssrc ssrc_b);
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_SSRC_H_