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.cc

@@ -0,0 +1,25 @@
+// 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.
+
+#include "cast/streaming/impl/rtcp_session.h"
+
+#include "util/osp_logging.h"
+
+namespace openscreen::cast {
+
+RtcpSession::RtcpSession(Ssrc sender_ssrc,
+                         Ssrc receiver_ssrc,
+                         Clock::time_point start_time)
+    : sender_ssrc_(sender_ssrc),
+      receiver_ssrc_(receiver_ssrc),
+      start_time_(start_time),
+      ntp_converter_(start_time) {
+  OSP_CHECK_NE(sender_ssrc_, kNullSsrc);
+  OSP_CHECK_NE(receiver_ssrc_, kNullSsrc);
+  OSP_CHECK_NE(sender_ssrc_, receiver_ssrc_);
+}
+
+RtcpSession::~RtcpSession() = default;
+
+}  // namespace openscreen::cast