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/message_constants.h

@@ -0,0 +1,15 @@
+// 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 CAST_STREAMING_IMPL_MESSAGE_CONSTANTS_H_
+#define CAST_STREAMING_IMPL_MESSAGE_CONSTANTS_H_
+
+namespace openscreen::cast {
+
+// RTP extension strings.
+inline constexpr char kInputEventsRtpExtension[] = "input_events";
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_IMPL_MESSAGE_CONSTANTS_H_