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.h

@@ -0,0 +1,33 @@
+// Copyright 2018 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UTIL_STRINGPRINTF_H_
+#define UTIL_STRINGPRINTF_H_
+
+#include <stdint.h>
+
+#include <format>
+#include <ostream>
+#include <string>
+#include <utility>
+
+#include "platform/base/span.h"
+
+namespace openscreen {
+
+// TODO(crbug.com/364687926): remove and replace with direct calls to
+// std::format now that we are on C++20.
+template <typename... Args>
+[[nodiscard]] std::string StringFormat(std::format_string<Args...> fmt,
+                                       Args&&... args) {
+  return std::format(fmt, std::forward<Args>(args)...);
+}
+
+// Returns a hex string representation of the given `bytes`.
+std::string HexEncode(const uint8_t* bytes, size_t len);
+std::string HexEncode(ByteView bytes);
+
+}  // namespace openscreen
+
+#endif  // UTIL_STRINGPRINTF_H_