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/base64.h

@@ -0,0 +1,32 @@
+// 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.
+
+#ifndef UTIL_BASE64_H_
+#define UTIL_BASE64_H_
+
+#include <stdint.h>
+
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include "platform/base/error.h"
+#include "platform/base/span.h"
+
+namespace openscreen::base64 {
+
+// Encodes the input binary data in base64.
+std::string Encode(ByteView input);
+
+// Encodes the input string in base64.
+std::string Encode(std::string_view input);
+
+// Decodes the base64 input string.  Returns true if successful and false
+// otherwise. The output string is only modified if successful. The decoding can
+// be done in-place.
+bool Decode(std::string_view input, std::vector<uint8_t>* output);
+
+}  // namespace openscreen::base64
+
+#endif  // UTIL_BASE64_H_