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/stringprintf.cc

@@ -0,0 +1,29 @@
+// Copyright 2020 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/stringprintf.h"
+
+#include <cstdarg>
+#include <cstdio>
+#include <iomanip>
+#include <sstream>
+
+#include "util/osp_logging.h"
+
+namespace openscreen {
+
+std::string HexEncode(const uint8_t* bytes, size_t len) {
+  return HexEncode(ByteView(bytes, len));
+}
+
+std::string HexEncode(ByteView bytes) {
+  std::ostringstream hex_dump;
+  hex_dump << std::setfill('0') << std::hex;
+  for (uint8_t byte : bytes) {
+    hex_dump << std::setw(2) << static_cast<int>(byte);
+  }
+  return hex_dump.str();
+}
+
+}  // namespace openscreen