"Are contracts built in to the language on a fundamental level, or are they just part of the standard library? "
One of the points of Lisp is that that difference doesn't matter.
But besides that, in this case it couldn't possibly matter. You write contracts for your functions. If they're violated at runtime you'll get a clear error that stops execution to contain damage, and assigns blame to the contract violator. At what point in that process does it matter whether contracts are "built into the language"?