Fix MSVC build of callback latency target

[Copilot]

The Windows Release build failed while compiling sfFDN.callback_latency: MSVC does not accept std::string_view iterators for std::from_chars.

• MSVC portability
  • Pass the required character-pointer range to std::from_chars.
  • Compare the returned pointer against the corresponding pointer end.

const auto [end, error] =
    std::from_chars(value.data(), value.data() + value.size(), iterations);