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/sender_message.h

@@ -0,0 +1,55 @@
+// Copyright 2020 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_SENDER_MESSAGE_H_
+#define CAST_STREAMING_SENDER_MESSAGE_H_
+
+#include <memory>
+#include <string>
+#include <variant>
+#include <vector>
+
+#include "cast/streaming/public/offer_messages.h"
+#include "json/value.h"
+#include "platform/base/error.h"
+#include "util/osp_logging.h"
+
+namespace openscreen::cast {
+
+struct SenderMessage {
+ public:
+  // Receiver response message type.
+  enum class Type {
+    // Unknown message type.
+    kUnknown,
+
+    // OFFER request message.
+    kOffer,
+
+    // GET_CAPABILITIES request message.
+    kGetCapabilities,
+
+    // Rpc binary messages. The payload is base64-encoded.
+    kRpc,
+
+    // Input-related binary messages. The payload is base64-encoded.
+    kInput,
+  };
+
+  static ErrorOr<SenderMessage> Parse(const Json::Value& value);
+  ErrorOr<Json::Value> ToJson() const;
+
+  Type type = Type::kUnknown;
+  int32_t sequence_number = -1;
+  bool valid = false;
+  std::variant<std::monostate,
+               std::vector<uint8_t>,  // Binary-encoded protobuf message.
+               Offer,
+               std::string>
+      body;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_SENDER_MESSAGE_H_