fmt 12.2 changed the implementation path used by `fmt::format_to` with
`std::back_insert_iterator`. Under nvcc, that path rejects
`std::back_insert_iterator<fmt::memory_buffer>` because fmt’s constexpr helper
takes a non-literal iterator type. Replacing `std::back_inserter(buffer)` with
`fmt::appender(buffer)` for `fmt::memory_buffer` outputs avoids that path and is
the fmt-native way to append to `memory_buffer`.
