在 'JsonParser::consumeUnicode' 中进行的 'char' 转换

Requires the implicit-conversion checks, which `-fsanitize=undefined` does not include. Build as in the `bittorrent::processRootDictionary` case (`-fsanitize=implicit-unsigned-integer-truncation,implicit-signed-integer-truncation,implicit-integer-sign-change` plus the ignorelist that silences `bitfield.h`, `base64.h`, the `util.h` byte helpers and `deps/`). With that ignorelist a well-formed run reports nothing, so every hit is signal.