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.cc

@@ -0,0 +1,39 @@
+// 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 "cast/streaming/impl/packet_util.h"
+
+#include "cast/streaming/impl/rtcp_common.h"
+#include "cast/streaming/impl/rtp_defines.h"
+
+namespace openscreen::cast {
+
+std::pair<ApparentPacketType, Ssrc> InspectPacketForRouting(ByteView packet) {
+  // Check for RTP packets first, since they are more frequent.
+  if (packet.size() >= kRtpPacketMinValidSize &&
+      packet[0] == kRtpRequiredFirstByte &&
+      IsRtpPayloadType(packet[1] & kRtpPayloadTypeMask)) {
+    constexpr int kOffsetToSsrcField = 8;
+    return std::make_pair(
+        ApparentPacketType::RTP,
+        Ssrc{ReadBigEndian<uint32_t>(packet.data() + kOffsetToSsrcField)});
+  }
+
+  // While RTCP packets are valid if they consist of just the RTCP Common
+  // Header, all the RTCP packet types processed by this implementation will
+  // also have a SSRC field immediately following the header. This is important
+  // for routing the packet to the correct parser instance.
+  constexpr int kRtcpPacketMinAcceptableSize =
+      kRtcpCommonHeaderSize + sizeof(uint32_t);
+  if (packet.size() >= kRtcpPacketMinAcceptableSize &&
+      RtcpCommonHeader::Parse(packet).has_value()) {
+    return std::make_pair(
+        ApparentPacketType::RTCP,
+        Ssrc{ReadBigEndian<uint32_t>(packet.data() + kRtcpCommonHeaderSize)});
+  }
+
+  return std::make_pair(ApparentPacketType::UNKNOWN, Ssrc{0});
+}
+
+}  // namespace openscreen::cast