Introduced me to the tailrec modifier, interesting.

I'm just writing a comment to see how my honor changes. And your code is great!

It somehow worked for the platform environment, wereas it's not working for my local environment

I think this should work: https://junit.org/junit5/docs/current/user-guide/#writing-tests-test-execution-order

Sadly, there isn't

Is there an annotation or something to run tests in order? It would be better if big tests were last.

Approved

Makes sense. I've added a failure message. It'll look like this:
input num: 2140 expected:<MMCXL[]> but was:<MMCXL[I]>

I would add the input in the random tests for better user experience while debugging:
something like this ->

assertEquals(ref(it), encode(it), "input: " + it)

Approved

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