Cosmo, ‘what legally matters’ is the judgement of the investigation after an event; there is no ‘legality’ beforehand, as the process depends on an adverse outcome.
Exactly. Hence, since no sane responsible person would choose declare a wet runway dry, there will be nothing to prove since an adverse outcome is not likely to occur.
Should a rejected take off end in an overrun for other unforeseeable reasons, blackbox performance data would still back up the correct decision to deem a dry runway dry - regardless of what is stated on the ATIS.
Or if you are particularly paranoid, grab your iPhone at take a picture before the start of the take off roll.
