SIL: avoid truncation warnings on Windows (NFCI)
`enum` is a signed integer type on some platforms. The return value of
`std::numeric_limits<T>::max()` is a `size_t` which is unsigned. Further
manipulation retains the unsigned-ness until the assignment which
results in truncation. Change the type to a `constexpr` constant
definition and remove the `enum` type along with a type conversion to an
explicitly unsigned integral type.
