You could add a c file with some unittests.

I had a bug:

    u_printf( my_output_cb, NULL, "%d" NL, -42 );

--42

I may have found an explanation. In u_parse_format(), case 'i': calls u_itoa( value, buffer, 10, false ); which outputs minus sign.

Then if ( number ) does if ( sign ) { output_cb( '-', ctx ); chars_written++; } which outputs another minus sign.

Is it really possible to reconcile "no bloat" with "fully standard compliant"?

Maybe have a look at https://github.com/eyalroz/printf, if only for the test suite, which looks pretty comprehensive and still fails.

Maybe "key features" does not mean the same in all projects, making every project appear "unnecessarily bloated while missing key features" in every other context?

I encourage you to continue working on your projects.