Add a new enum type with uint64_t as the underlying type.
Use it in the overall UDT. Not strictly needed, but it helps exercise its expected usage.
Create an object of this enum type with a large value (negative if cast to int64_t).
Perform several checks on this object as converted to `json`, which fail without the fix.

Select the correct json type depending on the signedness of the enum's underlying type.
This fixes the new checks in the unit test.

I ran `make pretty` but that modified 20 files, performing a significant amount of indentation changes, none of them related to my change.
I ran `make amalgamate`, but that did nothing. Apparently, the make rule won't run if the single_include files have already been updated by `make pretty`.
I forced `make amalgamate` to do the work by touching the file with the fix.
I then decided to keep just the minimal needed change: the addition of the fix to the single_include file.

I just am not conversant enough in Linux to know whether I installed astyle correctly (had to clone the source from a beta branch and build, in order to get support for `--squeeze-lines`).

The fix was relying on implicit conversions in the non-taken branch.
- Ordinarily (work on a C++20 codebase) I would have used `if constexpr` here, sidestepping the issue, but that's not available on C++11 so I didn't bother.
- So instead of an `if` statement, I used a compile-time constant to select the correct overload.
- This is arguably better in this case, anyway.

I was using function-style casts for typed constants, which I consider superior for constants, but the CI checks disagree, so changed all to `static_cast`.
- For some reason, the CI checks didn't point at all of them, so I hope I caught them all myself.

Built with clang14 and all unit tests pass.