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/platform/base/span.h

@@ -0,0 +1,39 @@
+// Copyright 2023 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef PLATFORM_BASE_SPAN_H_
+#define PLATFORM_BASE_SPAN_H_
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include <array>
+#include <cassert>
+#include <span>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "platform/base/type_util.h"
+
+namespace openscreen {
+
+// In Open Screen code, use these aliases for the most common types of Spans.
+// TODO(crbug.com/364687926): rename to byte_view.h and remove Span alias.
+using ByteView = std::span<const uint8_t>;
+using ByteBuffer = std::span<uint8_t>;
+template <typename T>
+using Span = std::span<T>;
+
+inline ByteView ByteViewFromString(std::string_view str) {
+  return ByteView(reinterpret_cast<const uint8_t*>(str.data()), str.size());
+}
+
+inline std::string ByteViewToString(ByteView bytes) {
+  return std::string(reinterpret_cast<const char*>(bytes.data()), bytes.size());
+}
+
+}  // namespace openscreen
+
+#endif  // PLATFORM_BASE_SPAN_H_