I don't know of any official confirmation, but forum consensus seems to be that the amount of damage dealt is not constrained by the HP of the target, so Shockwave and Martyr both count overkill. Granted, the person who I remember most recently pointing this out was Foote, and I seem to recall you two tending to disagree on pretty much everything. But in general, the safest assumption seems to be that there are no "invisible rules" other than those in the Glossary (eg handling of duplicate Limited cards) or on the official website (eg shuffling your deck)...if you're mentally inserting a rule such as "damage dealt to a target cannot exceed its current hit points", where no such rule is explicitly stated, you're probably doing so in error.
I'm not inserting a rule, the ruling that allows for -hp states that only in Certain Specific situations can a target go into negative HP. We seem to have interpretted that to mean things always go into negative HP if they take more damage than they have hp, which doesn't seem like the intent of the ruling at all.
Targets can go to negative HP, the original Baron Blade's flip side says "When Baron Blade is reduce to 0 or fewer HP". Can't get much more official than the original villain!
a LOT of cards have that exact wording. Overkill exists.
Over-healing, however, does not. So if you were supposed to regain 2 hp, but you only regain 1 because you hit your max, you only get to Mortal Form for 1, not 2.
Somewhat unfair, that. Makes playing the Scholar much trickier, and it just seems out of character for him to be tricky. He's supposed to be taking it easy...why does he sometimes say "please hurt me a bunch more so that I don't max out on healing"? Doesn't seem...copacetic, somehow.
One of my favorite things with scholar is Fanatic and Legacy with him, get the +damage and embolden on Scholar and Scholar can rock all day long without ever using solid to liquid, even if the villain isn't hitting him.