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/util/json/json_serialization.cc

@@ -0,0 +1,62 @@
+// 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 "util/json/json_serialization.h"
+
+#include <memory>
+#include <sstream>
+#include <string>
+#include <utility>
+
+#include "json/reader.h"
+#include "json/writer.h"
+#include "platform/base/error.h"
+#include "util/osp_logging.h"
+
+namespace openscreen::json {
+
+ErrorOr<Json::Value> Parse(std::string_view document) {
+  Json::CharReaderBuilder builder;
+  Json::CharReaderBuilder::strictMode(&builder.settings_);
+  if (document.empty()) {
+    return ErrorOr<Json::Value>(Error::Code::kJsonParseError, "empty document");
+  }
+
+  Json::Value root_node;
+  std::string error_msg;
+  std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
+  const bool succeeded = reader->parse(&*document.begin(), &*document.end(),
+                                       &root_node, &error_msg);
+  if (!succeeded) {
+    return ErrorOr<Json::Value>(Error::Code::kJsonParseError, error_msg);
+  }
+
+  return root_node;
+}
+
+ErrorOr<std::string> Stringify(const Json::Value& value) {
+  Json::StreamWriterBuilder factory;
+#ifndef _DEBUG
+  // Default is to "pretty print" the output JSON in a human readable
+  // format. On non-debug builds, we can remove pretty printing by simply
+  // getting rid of all indentation.
+  factory["indentation"] = "";
+#endif
+
+  std::unique_ptr<Json::StreamWriter> const writer(factory.newStreamWriter());
+  std::ostringstream stream;
+  writer->write(value, &stream);
+
+  if (!stream) {
+    // Note: jsoncpp doesn't give us more information about what actually
+    // went wrong, just says to "check the stream". However, failures on
+    // the stream should be rare, as we do not throw any errors in the jsoncpp
+    // library.
+    return ErrorOr<std::string>(Error::Code::kJsonWriteError, "Invalid stream");
+  }
+
+  return stream.str();
+}
+
+}  // namespace openscreen::json