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/util/big_endian.cc

@@ -0,0 +1,47 @@
+// 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.
+
+#include "util/big_endian.h"
+
+namespace openscreen {
+
+BigEndianReader::BigEndianReader(ByteView buffer) : BigEndianBuffer(buffer) {}
+
+BigEndianReader::BigEndianReader(const uint8_t* buffer, size_t length)
+    : BigEndianBuffer(buffer, length) {}
+
+bool BigEndianReader::Read(size_t length, void* out) {
+  return Read(ByteBuffer(static_cast<uint8_t*>(out), length));
+}
+
+bool BigEndianReader::Read(ByteBuffer out) {
+  ByteView view = remaining_span();
+  if (view.size() >= out.size()) {
+    std::copy(view.begin(), view.begin() + out.size(), out.begin());
+    Skip(out.size());
+    return true;
+  }
+  return false;
+}
+
+BigEndianWriter::BigEndianWriter(ByteBuffer buffer) : BigEndianBuffer(buffer) {}
+
+BigEndianWriter::BigEndianWriter(uint8_t* buffer, size_t length)
+    : BigEndianBuffer(buffer, length) {}
+
+bool BigEndianWriter::Write(const void* buffer, size_t length) {
+  return Write(ByteView(static_cast<const uint8_t*>(buffer), length));
+}
+
+bool BigEndianWriter::Write(ByteView buffer) {
+  ByteBuffer view = remaining_span();
+  if (view.size() >= buffer.size()) {
+    std::copy(buffer.begin(), buffer.end(), view.begin());
+    Skip(buffer.size());
+    return true;
+  }
+  return false;
+}
+
+}  // namespace openscreen