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/public/receiver_message.h

@@ -0,0 +1,117 @@
+// 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_PUBLIC_RECEIVER_MESSAGE_H_
+#define CAST_STREAMING_PUBLIC_RECEIVER_MESSAGE_H_
+
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <variant>
+#include <vector>
+
+#include "cast/streaming/public/answer_messages.h"
+#include "json/value.h"
+#include "util/osp_logging.h"
+
+namespace openscreen::cast {
+
+enum class MediaCapability {
+  kAudio,
+  kAac,
+  kOpus,
+  kVideo,
+  k4k,
+  kH264,
+  kVp8,
+  kVp9,
+  kHevc,
+  kAv1
+};
+
+struct ReceiverCapability {
+  static constexpr int kRemotingVersionUnknown = -1;
+
+  Json::Value ToJson() const;
+  static ErrorOr<ReceiverCapability> Parse(const Json::Value& value);
+
+  // The remoting version that the receiver uses.
+  int remoting_version = kRemotingVersionUnknown;
+
+  // Set of capabilities (e.g., ac3, 4k, hevc, vp9, dolby_vision, etc.).
+  std::vector<MediaCapability> media_capabilities;
+};
+
+// To avoid collisions with legacy error values, all Open Screen receiver errors
+// are offset.
+struct ReceiverError {
+  explicit ReceiverError(int code, std::string_view description = "");
+  explicit ReceiverError(Error::Code code, std::string_view description = "");
+  explicit ReceiverError(const Error& error);
+
+  ReceiverError(const ReceiverError&);
+  ReceiverError(ReceiverError&&) noexcept;
+  ReceiverError& operator=(const ReceiverError&);
+  ReceiverError& operator=(ReceiverError&&);
+  ~ReceiverError();
+
+  Json::Value ToJson() const;
+  static ErrorOr<ReceiverError> Parse(const Json::Value& value);
+  Error ToError() const;
+
+  // All Open Screen errors are offset by a fixed value to avoid overlapping
+  // with legacy values.
+  static constexpr int kOpenscreenErrorOffset = 10000;
+
+  // Raw error code.
+  int32_t code = -1;
+
+  // Parsed openscreen::Error code. May be nullopt if not a match.
+  std::optional<Error::Code> openscreen_code;
+
+  // Error description.
+  std::string description;
+};
+
+struct ReceiverMessage {
+ public:
+  // Receiver response message type.
+  enum class Type {
+    // Unknown message type.
+    kUnknown,
+
+    // Response to OFFER message.
+    kAnswer,
+
+    // Response to GET_CAPABILITIES message.
+    kCapabilitiesResponse,
+
+    // Rpc binary messages. The payload is base64-encoded.
+    kRpc,
+
+    // Input-related binary messages. The payload is base64-encoded.
+    kInput,
+  };
+
+  static ErrorOr<ReceiverMessage> 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,
+               Answer,
+               std::vector<uint8_t>,  // Binary-encoded protobuf message.
+               ReceiverCapability,
+               ReceiverError>
+      body;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_PUBLIC_RECEIVER_MESSAGE_H_