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/platform/impl/socket_handle.h

@@ -0,0 +1,27 @@
+// 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 PLATFORM_IMPL_SOCKET_HANDLE_H_
+#define PLATFORM_IMPL_SOCKET_HANDLE_H_
+
+#include <cstdlib>
+
+namespace openscreen {
+
+// A SocketHandle is the handle used to access a Socket by the underlying
+// platform.
+struct SocketHandle;
+
+struct SocketHandleHash {
+  size_t operator()(const SocketHandle& handle) const;
+};
+
+bool operator==(const SocketHandle& lhs, const SocketHandle& rhs);
+inline bool operator!=(const SocketHandle& lhs, const SocketHandle& rhs) {
+  return !(lhs == rhs);
+}
+
+}  // namespace openscreen
+
+#endif  // PLATFORM_IMPL_SOCKET_HANDLE_H_