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/common/public/message_port.h

@@ -0,0 +1,52 @@
+// 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_COMMON_PUBLIC_MESSAGE_PORT_H_
+#define CAST_COMMON_PUBLIC_MESSAGE_PORT_H_
+
+#include <string>
+
+#include "platform/base/error.h"
+
+namespace openscreen::cast {
+
+// This interface is intended to provide an abstraction for communicating
+// cast messages across a pipe with guaranteed delivery. This is used to
+// decouple the cast streaming receiver and sender sessions from the
+// network implementation.
+class MessagePort {
+ public:
+  class Client {
+   public:
+    // Called whenever a message arrives on the message port.
+    virtual void OnMessage(const std::string& source_id,
+                           const std::string& message_namespace,
+                           const std::string& message) = 0;
+
+    // Called whenever an error occurs on the message port.
+    virtual void OnError(const Error& error) = 0;
+
+    // Clients should expose a unique identifier used as the "source" of
+    // all messages sent on this message port.
+    virtual const std::string& source_id() = 0;
+
+   protected:
+    virtual ~Client() = default;
+  };
+
+  virtual ~MessagePort() = default;
+
+  // Set or reset the `MessagePort::Client` for this instance.
+  virtual void SetClient(Client& client) = 0;
+  virtual void ResetClient() = 0;
+
+  // Sends a message to a given `destination_id`.
+  virtual void PostMessage(const std::string& destination_id,
+                           const std::string& message_namespace,
+                           const std::string& message) = 0;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_COMMON_PUBLIC_MESSAGE_PORT_H_