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/impl/rtcp_session.h

@@ -0,0 +1,43 @@
+// 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_IMPL_RTCP_SESSION_H_
+#define CAST_STREAMING_IMPL_RTCP_SESSION_H_
+
+#include "cast/streaming/impl/ntp_time.h"
+#include "cast/streaming/ssrc.h"
+
+namespace openscreen::cast {
+
+// Session-level configuration and shared components for the RTCP messaging
+// associated with a single Cast RTP stream. Multiple packet serialization and
+// parsing components share a single RtcpSession instance for data consistency.
+class RtcpSession {
+ public:
+  // `start_time` should be the current time, as it is used by NtpTimeConverter
+  // to set a fixed reference point between the local Clock and current "real
+  // world" wall time.
+  RtcpSession(Ssrc sender_ssrc,
+              Ssrc receiver_ssrc,
+              Clock::time_point start_time);
+  ~RtcpSession();
+
+  Ssrc sender_ssrc() const { return sender_ssrc_; }
+  Ssrc receiver_ssrc() const { return receiver_ssrc_; }
+  const NtpTimeConverter& ntp_converter() const { return ntp_converter_; }
+  Clock::time_point start_time() const { return start_time_; }
+
+ private:
+  const Ssrc sender_ssrc_;
+  const Ssrc receiver_ssrc_;
+
+  Clock::time_point start_time_;
+
+  // Translates between system time (internal format) and NTP (wire format).
+  NtpTimeConverter ntp_converter_;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_IMPL_RTCP_SESSION_H_