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/answer_messages.h

@@ -0,0 +1,122 @@
+// 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_ANSWER_MESSAGES_H_
+#define CAST_STREAMING_PUBLIC_ANSWER_MESSAGES_H_
+
+#include <array>
+#include <chrono>
+#include <cstdint>
+#include <initializer_list>
+#include <memory>
+#include <optional>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "cast/streaming/resolution.h"
+#include "cast/streaming/ssrc.h"
+#include "json/value.h"
+#include "platform/base/error.h"
+#include "util/simple_fraction.h"
+
+namespace openscreen::cast {
+
+// For each of the below classes, though a number of methods are shared, the use
+// of a shared base class has intentionally been avoided. This is to improve
+// readability of the structs provided in this file by cutting down on the
+// amount of obscuring boilerplate code. For each of the following struct
+// definitions, the following method definitions are shared:
+// (1) TryParse. Shall return a boolean indicating whether the out
+//     parameter is in a valid state after checking bounds and restrictions.
+// (2) ToJson. Should return a proper JSON object. Assumes that IsValid()
+//     has been called already, OSP_CHECKs if not IsValid().
+// (3) IsValid. Used by both TryParse and ToJson to ensure that the
+//     object is in a good state.
+struct AudioConstraints {
+  static ErrorOr<AudioConstraints> TryParse(const Json::Value& value);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  int max_sample_rate = 0;
+  int max_channels = 0;
+  int min_bit_rate = 0;  // optional
+  int max_bit_rate = 0;
+  std::optional<std::chrono::milliseconds> max_delay = {};
+};
+
+struct VideoConstraints {
+  static ErrorOr<VideoConstraints> TryParse(const Json::Value& value);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  std::optional<double> max_pixels_per_second = {};
+  std::optional<Dimensions> min_resolution = {};
+  Dimensions max_dimensions = {};
+  int min_bit_rate = 0;  // optional
+  int max_bit_rate = 0;
+  std::optional<std::chrono::milliseconds> max_delay = {};
+};
+
+struct Constraints {
+  static ErrorOr<Constraints> TryParse(const Json::Value& value);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  AudioConstraints audio;
+  VideoConstraints video;
+};
+
+// Decides whether the Sender scales and letterboxes content to 16:9, or if
+// it may send video frames of any arbitrary size and the Receiver must
+// handle the presentation details.
+enum class AspectRatioConstraint : uint8_t { kVariable = 0, kFixed };
+
+struct AspectRatio {
+  static ErrorOr<AspectRatio> TryParse(const Json::Value& value);
+  bool IsValid() const;
+
+  bool operator==(const AspectRatio& other) const {
+    return width == other.width && height == other.height;
+  }
+
+  int width = 0;
+  int height = 0;
+};
+
+struct DisplayDescription {
+  static ErrorOr<DisplayDescription> TryParse(const Json::Value& value);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  // May exceed, be the same, or less than those mentioned in the
+  // video constraints.
+  std::optional<Dimensions> dimensions;
+  std::optional<AspectRatio> aspect_ratio = {};
+  std::optional<AspectRatioConstraint> aspect_ratio_constraint = {};
+};
+
+struct Answer {
+  static ErrorOr<Answer> TryParse(const Json::Value& value);
+  Json::Value ToJson() const;
+  bool IsValid() const;
+
+  int udp_port = 0;
+  std::vector<int> send_indexes;
+  std::vector<Ssrc> ssrcs;
+
+  // Constraints and display descriptions are optional fields, and maybe null in
+  // the valid case.
+  std::optional<Constraints> constraints;
+  std::optional<DisplayDescription> display;
+  std::vector<int> receiver_rtcp_event_log;
+  std::vector<int> receiver_rtcp_dscp;
+
+  // RTP extensions should be empty, but not null.
+  std::vector<std::vector<std::string>> rtp_extensions = {};
+};
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_PUBLIC_ANSWER_MESSAGES_H_