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_value.h

@@ -0,0 +1,29 @@
+// 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 UTIL_JSON_JSON_VALUE_H_
+#define UTIL_JSON_JSON_VALUE_H_
+
+#include <optional>
+#include <string_view>
+
+#include "json/value.h"
+
+#define JSON_EXPAND_FIND_CONSTANT_ARGS(s) (s), ((s) + sizeof(s) - 1)
+
+namespace openscreen {
+
+std::optional<int> MaybeGetInt(const Json::Value& message,
+                               const char* first,
+                               const char* last);
+
+std::optional<std::string_view> MaybeGetString(const Json::Value& message);
+
+std::optional<std::string_view> MaybeGetString(const Json::Value& message,
+                                               const char* first,
+                                               const char* last);
+
+}  // namespace openscreen
+
+#endif  // UTIL_JSON_JSON_VALUE_H_