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/offer_messages.h

@@ -0,0 +1,115 @@
+// 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 CAST_STREAMING_PUBLIC_OFFER_MESSAGES_H_
+#define CAST_STREAMING_PUBLIC_OFFER_MESSAGES_H_
+
+#include <chrono>
+#include <string>
+#include <vector>
+
+#include "cast/streaming/impl/rtp_defines.h"
+#include "cast/streaming/message_fields.h"
+#include "cast/streaming/public/session_config.h"
+#include "cast/streaming/resolution.h"
+#include "json/value.h"
+#include "platform/base/error.h"
+#include "util/simple_fraction.h"
+
+// This file contains the implementation of the Cast V2 Mirroring Control
+// Protocol offer object definition.
+namespace openscreen::cast {
+
+// If the target delay provided by the sender is not bounded by
+// [kMinTargetDelay, kMaxTargetDelay], it will be set to
+// kDefaultTargetPlayoutDelay.
+inline constexpr auto kMinTargetPlayoutDelay = std::chrono::milliseconds(0);
+inline constexpr auto kMaxTargetPlayoutDelay = std::chrono::milliseconds(5000);
+
+// If the sender provides an invalid maximum frame rate, it ill
+// be set to kDefaultMaxFrameRate.
+inline constexpr int kDefaultMaxFrameRate = 30;
+
+inline constexpr int kDefaultNumVideoChannels = 1;
+inline constexpr int kDefaultNumAudioChannels = 2;
+
+// A stream, as detailed by the CastV2 protocol spec, is a segment of an
+// offer message specifically representing a configuration object for
+// a codec and its related fields, such as maximum bit rate, time base,
+// and other fields.
+// Composed classes include AudioStream and VideoStream, which contain
+// fields specific to audio and video respectively.
+struct Stream {
+  enum class Type : uint8_t { kAudioSource, kVideoSource };
+
+  static ErrorOr<Stream> TryParse(const Json::Value& root, Stream::Type type);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  int index = 0;
+  Type type = {};
+
+  // Default channel count is 1, e.g. for video.
+  int channels = 0;
+  RtpPayloadType rtp_payload_type = {};
+  Ssrc ssrc = {};
+  std::chrono::milliseconds target_delay = {};
+
+  // AES Key and IV mask format is very strict: a 32 digit hex string that
+  // must be converted to a 16 digit byte array.
+  std::array<uint8_t, 16> aes_key = {};
+  std::array<uint8_t, 16> aes_iv_mask = {};
+
+  // The event logs are generally recommended for use in gathering statistics
+  // for the sender session.
+  bool receiver_rtcp_event_log = true;
+  std::optional<int> receiver_rtcp_dscp;
+  int rtp_timebase = 0;
+
+  // The codec parameter field honors the format laid out in RFC 6381:
+  // https://datatracker.ietf.org/doc/html/rfc6381.
+  std::string codec_parameter;
+
+  std::vector<std::string> rtp_extensions;
+};
+
+struct AudioStream {
+  static ErrorOr<AudioStream> TryParse(const Json::Value& root);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  Stream stream;
+  AudioCodec codec = AudioCodec::kNotSpecified;
+  int bit_rate = 0;
+};
+
+struct VideoStream {
+  static ErrorOr<VideoStream> TryParse(const Json::Value& root);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  Stream stream;
+  VideoCodec codec = VideoCodec::kNotSpecified;
+  SimpleFraction max_frame_rate;
+  int max_bit_rate = 0;
+  std::string protection;
+  std::string profile;
+  std::string level;
+  std::vector<Resolution> resolutions;
+  std::string error_recovery_mode;
+};
+
+struct Offer {
+  static ErrorOr<Offer> TryParse(const Json::Value& root);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  CastMode cast_mode = CastMode::kMirroring;
+  std::vector<AudioStream> audio_streams;
+  std::vector<VideoStream> video_streams;
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_PUBLIC_OFFER_MESSAGES_H_