REJECTED vs EVICTED? Are they synonyms? if not, what's the difference?

They are synonyms, I used a different word to make easier to distinguish between AddProofStatus and ProofRegistrationResult but obviously it didn't work and caused more confusion.

AFAICT there is a 1-1 mapping between ProofRegistrationResult values and the message uses, so I think it would be ideal to not pass the message here but instead have a const defined next to the ProofRegistrationResult enum that defines a string message for each ProofRegistrationResult value.

I think this is cleaner than having unexplained string values throughout the function and eliminates the chances of a call to invalidate passing mismatching arguments.

There is a twist here: the invalidate lambda is taking a ProofRegistrationResult, but the function returns a ProofRegistrationState (via the reference parameter). The "state" is mostly the final status + the "result" enum (kind of an error code) + a couple message strings for normal and debug print.

The 1-1 matching here is intentional but not a mandatory thing, e.g. the TxValidationResult enum have a TX_CONSENSUS value that can accommodate several different error messages. I don't think there is a need to enforce this 1-1 relationship, otherwise it should be done at the parent class ValidationResult level which is out of scope for this diff.