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/read_file.cc

@@ -0,0 +1,34 @@
+// 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.
+
+#include "util/read_file.h"
+
+#include <stdio.h>
+
+namespace openscreen {
+
+std::string ReadEntireFileToString(std::string_view filename) {
+  FILE* file = fopen(filename.data(), "r");
+  if (file == nullptr) {
+    return {};
+  }
+  fseek(file, 0, SEEK_END);
+  long file_size = ftell(file);  // NOLINT
+  fseek(file, 0, SEEK_SET);
+  std::string contents(file_size, 0);
+  int bytes_read = 0;
+  while (bytes_read < file_size) {
+    size_t ret = fread(&contents[bytes_read], 1, file_size - bytes_read, file);
+    if (ret == 0 && ferror(file)) {
+      return {};
+    } else {
+      bytes_read += ret;
+    }
+  }
+  fclose(file);
+
+  return contents;
+}
+
+}  // namespace openscreen