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/impl/packet_util.h

@@ -0,0 +1,60 @@
+// 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_IMPL_PACKET_UTIL_H_
+#define CAST_STREAMING_IMPL_PACKET_UTIL_H_
+
+#include <utility>
+
+#include "cast/streaming/ssrc.h"
+#include "platform/base/span.h"
+#include "util/big_endian.h"
+#include "util/osp_logging.h"
+
+namespace openscreen::cast {
+
+// Reads a field from the start of the given span and advances the span to point
+// just after the field.
+template <typename Integer>
+inline Integer ConsumeField(ByteView& in) {
+  OSP_CHECK_GE(in.size(), sizeof(Integer));
+  const Integer result = ReadBigEndian<Integer>(in.data());
+  in = in.subspan(sizeof(Integer));
+  return result;
+}
+
+// Writes a field at the start of the given span and advances the span to point
+// just after the field.
+template <typename Integer>
+inline void AppendField(Integer value, ByteBuffer& out) {
+  WriteBigEndian<Integer>(value, out.data());
+  out = out.subspan(sizeof(Integer));
+}
+
+// Returns a bitmask for a field having the given number of bits. For example,
+// FieldBitmask<uint8_t>(5) returns 0b00011111.
+template <typename Integer>
+constexpr Integer FieldBitmask(unsigned field_size_in_bits) {
+  return (Integer{1} << field_size_in_bits) - 1;
+}
+
+// Reserves `num_bytes` from the beginning of the given span, returning the
+// reserved space.
+inline ByteBuffer ReserveSpace(int num_bytes, ByteBuffer& out) {
+  const ByteBuffer reserved = out.subspan(0, num_bytes);
+  out = out.subspan(num_bytes);
+  return reserved;
+}
+
+// Performs a quick-scan of the packet data for the purposes of routing it to an
+// appropriate parser. Identifies whether the packet is a RTP packet, RTCP
+// packet, or unknown; and provides the originator's SSRC. This only performs a
+// very quick scan of the packet data, and does not guarantee that a full parse
+// will later succeed.
+enum class ApparentPacketType { UNKNOWN, RTP, RTCP };
+std::pair<ApparentPacketType, Ssrc> InspectPacketForRouting(ByteView packet);
+
+}  // namespace openscreen::cast
+
+#endif  // CAST_STREAMING_IMPL_PACKET_UTIL_H_