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/udp_packet.h

@@ -0,0 +1,54 @@
+// 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 PLATFORM_BASE_UDP_PACKET_H_
+#define PLATFORM_BASE_UDP_PACKET_H_
+
+#include <stdint.h>
+
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "platform/base/ip_address.h"
+
+namespace openscreen {
+
+// A move-only std::vector of bytes that may not exceed the maximum possible
+// size of a UDP packet. Implicit copy construction/assignment is disabled to
+// prevent hidden copies (i.e., those not explicitly coded).
+class UdpPacket : public std::vector<uint8_t> {
+ public:
+  // C++14 vector constructors, sans Allocator foo, and no copy ctor.
+  UdpPacket();
+  explicit UdpPacket(size_type size, uint8_t fill_value = {});
+  template <typename InputIt>
+  UdpPacket(InputIt first, InputIt last) : std::vector<uint8_t>(first, last) {}
+  UdpPacket(std::initializer_list<uint8_t> init);
+  UdpPacket(const UdpPacket&) = delete;
+  UdpPacket(UdpPacket&& other) noexcept;
+
+  ~UdpPacket();
+
+  UdpPacket& operator=(UdpPacket&& other);
+  UdpPacket& operator=(const UdpPacket&) = delete;
+
+  const IPEndpoint& source() const { return source_; }
+  void set_source(IPEndpoint endpoint) { source_ = std::move(endpoint); }
+
+  const IPEndpoint& destination() const { return destination_; }
+  void set_destination(IPEndpoint endpoint) {
+    destination_ = std::move(endpoint);
+  }
+
+  static constexpr size_type kUdpMaxPacketSize = 1 << 16;
+
+ private:
+  IPEndpoint source_ = {};
+  IPEndpoint destination_ = {};
+};
+
+}  // namespace openscreen
+
+#endif  // PLATFORM_BASE_UDP_PACKET_H_