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/crypto/random_bytes.cc

@@ -0,0 +1,23 @@
+// 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/crypto/random_bytes.h"
+
+#include "openssl/rand.h"
+#include "util/osp_logging.h"
+
+namespace openscreen {
+
+std::array<uint8_t, 16> GenerateRandomBytes16() {
+  std::array<uint8_t, 16> result;
+  GenerateRandomBytes(result);
+  return result;
+}
+
+void GenerateRandomBytes(ByteBuffer out) {
+  // Working cryptography is mandatory for our library to run.
+  OSP_CHECK(RAND_bytes(out.data(), out.size()) == 1);
+}
+
+}  // namespace openscreen