在 'GroupId::expandUnique' 中,传递给 'unsigned char' 参数的 'char' 为已签名

Requires the implicit-conversion checks, which `-fsanitize=undefined` does not include. Build as in the #2381 (`-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.